A workaround to get model scaling into StereoKit

ScaleModelSK.cs

public class Program
{
    private static void Main(string[] _)
    {
        // Initialize StereoKit
        SKSettings settings = new()
        {
            appName = "ModelScaleSK",
            assetsFolder = "Assets",
        };

        if (!SK.Initialize(settings))
        {
            Environment.Exit(1);
        }

        Material volumeMat = Default.MaterialUIBox.Copy();
        volumeMat["border_size"] = 0.0f;
        volumeMat["border_affect_radius"] = 0.3f;

        var model = Model.FromMesh(Mesh.GenerateSphere(15 * U.cm), Material.Default);
        float modelScale = 1;
        Pose modelPose = new(Input.Head.ToMatrix().Transform(Vec3.Forward * 0.3f), Quat.Identity);
        SK.Run(() =>
        {
            Bounds modelBoundsScaled = model.Bounds * modelScale;
            if (UIExtension.HandleBegin($"Test", ref modelPose, modelBoundsScaled, ref modelScale))
            {
                Default.MeshCube.Draw(volumeMat, Matrix.TS(modelBoundsScaled.center, modelBoundsScaled.dimensions));
            }
            UI.HandleEnd();

            Hierarchy.Push(modelPose.ToMatrix(modelScale));
            model.Draw(Matrix.Identity);
            Hierarchy.Pop();
        });
    }

    /// <summary>
    /// Extends StereoKit UI capabilities.
    /// </summary>
    public static class UIExtension
    {
        /// <summary>
        /// Temporary dictionary to store scale information of the model being scaled by the user.
        /// </summary>
        private static readonly Dictionary<string, float> s_IdScalePairs = new();

        /// <summary>
        /// A modification of UI.HandleBegin, that allows for scaling 3D models through
        /// hand interaction.
        /// </summary>
        /// <param name="id">Name of the 3d model being manipulated.</param>
        /// <param name="pose">Pose of the object being manipulated.</param>
        /// <param name="handle">Bounds of the object being manipulated.</param>
        /// <param name="scale">Current scale of the object.</param>
        /// <param name="drawHandle">Flag to indicate whether to draw the bounds.</param>
        /// <param name="moveType">Defines how the handle follows the user's hands.</param>
        /// <returns>The original result from StereoKit's HandleBegin.</returns>
        public static bool HandleBegin(string id, ref Pose pose, Bounds handle, ref float scale,
            bool drawHandle = false, UIMove moveType = UIMove.Exact)
        {
            if (UI.HandleBegin(id, ref pose, handle, drawHandle, moveType))
            {
                if (moveType != UIMove.Exact)
                {
                    return true;
                }

                Hand leftHand = Input.Hand(Handed.Left);
                Hand rightHand = Input.Hand(Handed.Right);

                if (leftHand.IsTracked && rightHand.IsTracked
                       && leftHand.IsPinched && rightHand.IsPinched)
                {
                    if (leftHand.IsJustPinched || rightHand.IsJustPinched)
                    {
                        handle.center = pose.ToMatrix().Transform(handle.center);
                        if (handle.Contains(leftHand.pinchPt) && handle.Contains(rightHand.pinchPt))
                        {
                            s_IdScalePairs[id] = Vec3.Distance(leftHand.pinchPt, rightHand.pinchPt) / scale;
                        }
                    }
                    else if (s_IdScalePairs.ContainsKey(id))
                    {
                        float currentDist = Vec3.Distance(leftHand.pinchPt, rightHand.pinchPt);
                        scale = currentDist / s_IdScalePairs[id];
                    }
                }
                else if (s_IdScalePairs.ContainsKey(id))
                {
                    s_IdScalePairs.Remove(id);
                }

                return true;
            }

            if (s_IdScalePairs.ContainsKey(id))
            {
                s_IdScalePairs.Remove(id);
            }

            return false;
        }

        public static bool Handle(string id, ref Pose pose, Bounds handle, ref float scale,
            bool drawHandle = false, UIMove moveType = UIMove.Exact)
        {
            bool result = HandleBegin(id, ref pose, handle, ref scale, drawHandle, moveType);
            UI.HandleEnd();
            return result;
        }
    }
}