A script to fix the horizontal FOV/orthographic size of a camera to a specified value in Unity

HorizontalCamera.cs

using UnityEngine;

[ExecuteInEditMode]
[RequireComponent( typeof( Camera ) )]
public class HorizontalCamera : MonoBehaviour
{
	private Camera m_camera;
	private float lastAspect;

#pragma warning disable 0649
	[SerializeField]
	private float m_fieldOfView = 60f;
	public float FieldOfView
	{
		get { return m_fieldOfView; }
		set
		{
			if( m_fieldOfView != value )
			{
				m_fieldOfView = value;
				RefreshCamera();
			}
		}
	}

	[SerializeField]
	private float m_orthographicSize = 5f;
	public float OrthographicSize
	{
		get { return m_orthographicSize; }
		set
		{
			if( m_orthographicSize != value )
			{
				m_orthographicSize = value;
				RefreshCamera();
			}
		}
	}
#pragma warning restore 0649

	private void OnEnable()
	{
		RefreshCamera();

#if UNITY_EDITOR
		UnityEditor.EditorApplication.update -= Update;
		UnityEditor.EditorApplication.update += Update;
#endif
	}

	private void Update()
	{
		float aspect = m_camera.aspect;
		if( aspect != lastAspect )
			AdjustCamera( aspect );
	}

	public void RefreshCamera()
	{
		if( !m_camera )
			m_camera = GetComponent<Camera>();

		AdjustCamera( m_camera.aspect );
	}

	private void AdjustCamera( float aspect )
	{
		lastAspect = aspect;

		// Credit: https://forum.unity.com/threads/how-to-calculate-horizontal-field-of-view.16114/#post-2961964
		float _1OverAspect = 1f / aspect;
		m_camera.fieldOfView = 2f * Mathf.Atan( Mathf.Tan( m_fieldOfView * Mathf.Deg2Rad * 0.5f ) * _1OverAspect ) * Mathf.Rad2Deg;
		m_camera.orthographicSize = m_orthographicSize * _1OverAspect;
	}

#if UNITY_EDITOR
	private void OnValidate()
	{
		RefreshCamera();
	}

	private void OnDisable()
	{
		UnityEditor.EditorApplication.update -= Update;
	}
#endif
}

Thanks for the script, Yasir. Here's my modified version for cinemachine camera

using Cinemachine;
using UnityEngine;

[ExecuteInEditMode]
[RequireComponent(typeof(CinemachineVirtualCamera))]
public class HorizontalCinemachineCamera : MonoBehaviour
{
    private CinemachineVirtualCamera m_camera;
    private float lastAspect;

#pragma warning disable 0649
    [SerializeField]
    private float m_fieldOfView = 60f;
    public float FieldOfView
    {
        get { return m_fieldOfView; }
        set
        {
            if (m_fieldOfView != value)
            {
                m_fieldOfView = value;
                RefreshCamera();
            }
        }
    }

    [SerializeField]
    private float m_orthographicSize = 5f;
    public float OrthographicSize
    {
        get { return m_orthographicSize; }
        set
        {
            if (m_orthographicSize != value)
            {
                m_orthographicSize = value;
                RefreshCamera();
            }
        }
    }
#pragma warning restore 0649

    private void OnEnable()
    {
        RefreshCamera();

#if UNITY_EDITOR
        UnityEditor.EditorApplication.update -= Update;
        UnityEditor.EditorApplication.update += Update;
#endif
    }

    private void Update()
    {
        float aspect = m_camera.m_Lens.Aspect;
        if (aspect != lastAspect)
            AdjustCamera(aspect);
    }

    public void RefreshCamera()
    {
        if (!m_camera)
            m_camera = GetComponent<CinemachineVirtualCamera>();

        AdjustCamera(m_camera.m_Lens.Aspect);
    }

    private void AdjustCamera(float aspect)
    {
        lastAspect = aspect;

        // Credit: https://forum.unity.com/threads/how-to-calculate-horizontal-field-of-view.16114/#post-2961964
        float _1OverAspect = 1f / aspect;
        m_camera.m_Lens.FieldOfView = 2f * Mathf.Atan(Mathf.Tan(m_fieldOfView * Mathf.Deg2Rad * 0.5f) * _1OverAspect) * Mathf.Rad2Deg;
        m_camera.m_Lens.OrthographicSize = m_orthographicSize * _1OverAspect;
    }

#if UNITY_EDITOR
    private void OnValidate()
    {
        RefreshCamera();
    }

    private void OnDisable()
    {
        UnityEditor.EditorApplication.update -= Update;
    }
#endif
}