An extension of Unity's LayoutElement script that enable more flexible adaptation such as matching another RecTransform's dimensions or setting maximum preferred values. Put the Inspector and Property Drawer scripts in an Editor folder. Also available as a package at https://github.com/QuantumCalzone/UnityLayoutElementExtended

LayoutElementExtended.cs

using UnityEngine;
using UnityEngine.UI;

namespace QuantumCalzone
{
    [AddComponentMenu("Layout/Extended/Layout Element Extended")]
    [RequireComponent(typeof(RectTransform))]
    public class LayoutElementExtended : LayoutElement
    {
        public LayoutElementExtendedValue MinWidthExtended = new LayoutElementExtendedValue();
        public LayoutElementExtendedValue MinHeightExtended = new LayoutElementExtendedValue();
        public LayoutElementExtendedValue PreferredWidthExtended = new LayoutElementExtendedValue();
        public LayoutElementExtendedValue PreferredHeightExtended = new LayoutElementExtendedValue();
        public LayoutElementExtendedValue FlexibleWidthExtended = new LayoutElementExtendedValue();
        public LayoutElementExtendedValue FlexibleHeightExtended = new LayoutElementExtendedValue();

        public LayoutElementExtendedValue MaxWidth = new LayoutElementExtendedValue();
        public LayoutElementExtendedValue MaxHeight = new LayoutElementExtendedValue();

        private RectTransform rectTransform = null;
        public RectTransform GetRectTransform {
            get {
                if (!rectTransform) rectTransform = GetComponent<RectTransform>();
                return rectTransform;
            }
        }

        public float Width {
            get {
                var valueToReturn = 0f;

                if (MinWidthExtended.Enabled && !PreferredWidthExtended.Enabled)
                {
                    valueToReturn = MinWidth;
                }
                else if (!MinWidthExtended.Enabled && PreferredWidthExtended.Enabled)
                {
                    valueToReturn = PreferredWidth;
                }
                else if (MinWidthExtended.Enabled && PreferredWidthExtended.Enabled)
                {
                    valueToReturn = PreferredWidth > MinWidth ? PreferredWidth : MinWidth;
                }

                return valueToReturn;
            }
        }

        public float Height {
            get {
                var valueToReturn = 0f;

                if (MinHeightExtended.Enabled && !PreferredHeightExtended.Enabled)
                {
                    valueToReturn = MinHeight;
                }
                else if (!MinHeightExtended.Enabled && PreferredHeightExtended.Enabled)
                {
                    valueToReturn = PreferredHeight;
                }
                else if (MinHeightExtended.Enabled && PreferredHeightExtended.Enabled)
                {
                    valueToReturn = PreferredHeight > MinHeight ? PreferredHeight : MinHeight;
                }

                return valueToReturn;
            }
        }

        #region Min

        private float MinWidth {
            get {
                var valueToReturn = 0f;

                if (MinWidthExtended.Enabled)
                {
                    MinWidthExtended.ProcessTargetValue(gameObject);

                    valueToReturn = MinWidthExtended.TargetValue;

                    if (MaxWidth.Enabled)
                    {
                        MaxWidth.ProcessTargetValue(gameObject);

                        if (valueToReturn > MaxWidth.TargetValue)
                            valueToReturn = MaxWidth.TargetValue;
                    }
                }

                return valueToReturn;
            }
        }

        private float MinHeight {
            get {
                var valueToReturn = 0f;

                if (MinHeightExtended.Enabled)
                {
                    MinHeightExtended.ProcessTargetValue(gameObject);

                    valueToReturn = MinHeightExtended.TargetValue;

                    if (MaxHeight.Enabled)
                    {
                        MaxHeight.ProcessTargetValue(gameObject);

                        if (valueToReturn > MaxHeight.TargetValue)
                            valueToReturn = MaxHeight.TargetValue;
                    }
                }

                return valueToReturn;
            }
        }

        #endregion

        #region Preferred

        private float PreferredWidth {
            get {
                var preferredWidth = 0f;

                if (PreferredWidthExtended.Enabled)
                {
                    PreferredWidthExtended.ProcessTargetValue(gameObject);

                    preferredWidth = PreferredWidthExtended.TargetValue;

                    if (MaxWidth.Enabled)
                    {
                        MaxWidth.ProcessTargetValue(gameObject);

                        if (preferredWidth > MaxWidth.TargetValue)
                            preferredWidth = MaxWidth.TargetValue;
                    }
                }

                return preferredWidth;
            }
        }

        private float PreferredHeight {
            get {
                var preferredHeight = 0f;

                if (PreferredHeightExtended.Enabled)
                {
                    PreferredHeightExtended.ProcessTargetValue(gameObject);

                    preferredHeight = PreferredHeightExtended.TargetValue;

                    if (MaxHeight.Enabled)
                    {
                        MaxHeight.ProcessTargetValue(gameObject);

                        if (preferredHeight > MaxHeight.TargetValue)
                            preferredHeight = MaxHeight.TargetValue;
                    }
                }

                return preferredHeight;
            }
        }

        #endregion

        #region Flexible

        private float FlexibleWidth {
            get {
                var flexibleWidth = 0f;

                if (FlexibleWidthExtended.Enabled)
                {
                    FlexibleWidthExtended.ProcessTargetValue(gameObject);

                    flexibleWidth = FlexibleWidthExtended.TargetValue;
                }

                return flexibleWidth;
            }
        }

        private float FlexibleHeight {
            get {
                var flexibleHeight = 0f;

                if (FlexibleHeightExtended.Enabled)
                {
                    FlexibleHeightExtended.ProcessTargetValue(gameObject);

                    flexibleHeight = FlexibleHeightExtended.TargetValue;
                }

                return flexibleHeight;
            }
        }

        #endregion

        #region Unity Methods

        public override void CalculateLayoutInputHorizontal()
        {
            minWidth = MinWidthExtended.Enabled ? MinWidth : -1;
            preferredWidth = PreferredWidthExtended.Enabled ? PreferredWidth : -1;
            flexibleWidth = FlexibleWidthExtended.Enabled ? FlexibleWidth : -1;

            base.CalculateLayoutInputHorizontal();
        }

        public override void CalculateLayoutInputVertical()
        {
            minHeight = MinHeightExtended.Enabled ? MinHeight : -1;
            preferredHeight = PreferredHeightExtended.Enabled ? PreferredHeight : -1;
            flexibleHeight = FlexibleHeightExtended.Enabled ? FlexibleHeight : -1;

            base.CalculateLayoutInputVertical();
        }
#if UNITY_EDITOR
        private void Update()
        {
            CalculateLayoutInputHorizontal();
            CalculateLayoutInputVertical();
        }
#endif
        #endregion
    }
}

LayoutElementExtendedEditor.cs

using UnityEditor;
using UnityEditor.UI;

namespace QuantumCalzone
{
    [CanEditMultipleObjects]
    [CustomEditor(typeof(LayoutElementExtended), true)]
    public class LayoutElementExtendedEditor : LayoutElementEditor
    {
        private SerializedProperty m_IgnoreLayout = null;
        private SerializedProperty m_LayoutPriority = null;

        private SerializedProperty MinWidthExtended = null;
        private SerializedProperty MinHeightExtended = null;
        private SerializedProperty PreferredWidthExtended = null;
        private SerializedProperty PreferredHeightExtended = null;
        private SerializedProperty FlexibleWidthExtended = null;
        private SerializedProperty FlexibleHeightExtended = null;
        private SerializedProperty MaxWidth = null;
        private SerializedProperty MaxHeight = null;

        public override void OnInspectorGUI()
        {
            serializedObject.Update();

            EditorGUILayout.PropertyField(m_IgnoreLayout);

            if (!m_IgnoreLayout.boolValue)
            {
                EditorGUILayout.BeginVertical("Box");

                EditorGUILayout.LabelField("Extended", EditorStyles.boldLabel);

                DisplayPropertyFieldInBox(MinWidthExtended);
                DisplayPropertyFieldInBox(MinHeightExtended);
                DisplayPropertyFieldInBox(PreferredWidthExtended);
                DisplayPropertyFieldInBox(PreferredHeightExtended);
                DisplayPropertyFieldInBox(FlexibleWidthExtended);
                DisplayPropertyFieldInBox(FlexibleHeightExtended);
                DisplayPropertyFieldInBox(MaxWidth);
                DisplayPropertyFieldInBox(MaxHeight);

                EditorGUILayout.EndVertical();
            }

            EditorGUILayout.PropertyField(m_LayoutPriority);

            serializedObject.ApplyModifiedProperties();
        }

        protected override void OnEnable()
        {
            base.OnEnable();

            m_IgnoreLayout = serializedObject.FindProperty("m_IgnoreLayout");
            m_LayoutPriority = serializedObject.FindProperty("m_LayoutPriority");

            MinWidthExtended = serializedObject.FindProperty("MinWidthExtended");
            MinHeightExtended = serializedObject.FindProperty("MinHeightExtended");
            PreferredWidthExtended = serializedObject.FindProperty("PreferredWidthExtended");
            PreferredHeightExtended = serializedObject.FindProperty("PreferredHeightExtended");
            FlexibleWidthExtended = serializedObject.FindProperty("FlexibleWidthExtended");
            FlexibleHeightExtended = serializedObject.FindProperty("FlexibleHeightExtended");
            MaxWidth = serializedObject.FindProperty("MaxWidth");
            MaxHeight = serializedObject.FindProperty("MaxHeight");
        }

        private void DisplayPropertyFieldInBox(SerializedProperty target)
        {
            EditorGUILayout.BeginVertical("Box");
            EditorGUILayout.PropertyField(target);
            EditorGUILayout.EndVertical();
        }
    }
}

LayoutElementExtendedValue.cs

using System;
using UnityEngine;

namespace QuantumCalzone
{
    [Serializable]
    public class LayoutElementExtendedValue
    {
        public enum ReferenceTypes { None, Width, Height }

        public bool Enabled = false;
        public ReferenceTypes ReferenceType = ReferenceTypes.None;
        public RectTransform Reference = null;
        public float ReferenceDelta = 0;
        public float TargetValue = 0;

        public void ProcessTargetValue(UnityEngine.Object context)
        {
            if (!Reference && ReferenceType != ReferenceTypes.None)
            {
                Debug.Log("This needs a reference to process your target Layout values", context);
                return;
            }

            switch (ReferenceType)
            {
                case ReferenceTypes.None:
                    break;
                case ReferenceTypes.Width:
                    TargetValue = Reference.rect.width;
                    break;
                case ReferenceTypes.Height:
                    TargetValue = Reference.rect.height;
                    break;
                default:
                    Debug.LogError(string.Format("No support for a referenceType of: {0}", ReferenceType), context);
                    break;
            }

            TargetValue += ReferenceDelta;
        }
    }
}

LayoutElementExtendedValuePropertyDrawer.cs

using UnityEngine;
using UnityEditor;

namespace QuantumCalzone
{
    [CustomPropertyDrawer(typeof(LayoutElementExtendedValue))]
    public class LayoutElementExtendedValuePropertyDrawer : PropertyDrawer
    {
        public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
        {
            EditorGUI.BeginProperty(position, label, property);

            GUI.Label(position, label, EditorStyles.boldLabel);

            var enabled = property.FindPropertyRelative("Enabled");
            EditorGUILayout.PropertyField(enabled);

            if (enabled.boolValue)
            {
                var referenceType = property.FindPropertyRelative("ReferenceType");
                var reference = property.FindPropertyRelative("Reference");
                var referenceDelta = property.FindPropertyRelative("ReferenceDelta");
                var targetValue = property.FindPropertyRelative("TargetValue");

                EditorGUILayout.PropertyField(referenceType);

                if ((LayoutElementExtendedValue.ReferenceTypes)referenceType.enumValueIndex != LayoutElementExtendedValue.ReferenceTypes.None)
                {
                    EditorGUILayout.PropertyField(reference);
                    EditorGUILayout.PropertyField(referenceDelta);
                    GUI.enabled = false;
                    EditorGUILayout.PropertyField(targetValue);
                    GUI.enabled = true;
                }
                else EditorGUILayout.PropertyField(targetValue);
            }

            EditorGUI.EndProperty();
        }
    }
}

This stuff is golden, ought to be built into unity. For those who use this, the "DisplayPropertyFieldInBox()" methods seem to be missing, just replace with standard unity ui logic, e.g. "minWidthExtended.DisplayPropertyFieldInBox();" becomes "EditorGUILayout.PropertyField(minWidthExtended);". Also remove the reference to the interface "IRectTransformGetable".

This stuff is golden, ought to be built into unity. For those who use this, the "DisplayPropertyFieldInBox()" methods seem to be missing, just replace with standard unity ui logic, e.g. "minWidthExtended.DisplayPropertyFieldInBox();" becomes "EditorGUILayout.PropertyField(minWidthExtended);". Also remove the reference to the interface "IRectTransformGetable".

Thanks for your feedback! I cleaned up those old references I forgot to remove before.

This is amazing. Exactly what I needed.

Hey, thanks for this!
I have a question. Do I have to do something special to use MaxHeight?
It isn't working for me.

Hey, thanks for this!
I have a question. Do I have to do something special to use MaxHeight?
It isn't working for me.

Is Flexible Height enabled and/or is Preferred Height enabled and its resulting value at something greater than the Max Height?

Also, feel free to use the package version of this for easier installation. You can find it here.