Set an object width scale relative to a screen pixel value.

CameraHelper.cs

using UnityEngine;

public static class CameraHelper
{
	public static float GetPixelWidthToWorldScale(Camera camera, Vector3 worldTargetPosition, float pixelWidth)
	{
		return GetPixelWidthToWorldScale(camera, worldTargetPosition, pixelWidth, 1f);
		
	}// GetPixelWidthToWorldScale()
	
	public static float GetPixelWidthToWorldScale(Camera camera, Vector3 worldTargetPosition, float pixelWidth, float targetMeterWidth)
	{
		if(camera.orthographic)
		{
			Debug.LogWarning("GetWorldScale() doesn't work on orthographic camera at the moment !");
			
			return 1f;
		}
		
		float cameraAspectRatio = (float)camera.pixelWidth;
		cameraAspectRatio /= (float)camera.pixelHeight;
		
		float relativePositionDepth = camera.transform.InverseTransformPoint(worldTargetPosition).z;
		
		float ratioPixelWidth = pixelWidth;
		ratioPixelWidth /= (float)camera.pixelWidth;
		
		float size = Mathf.Tan(camera.fieldOfView * Mathf.Deg2Rad * 0.5f) * relativePositionDepth * 2f;
		
		return size * cameraAspectRatio * ratioPixelWidth / targetMeterWidth;
		
	}// GetPixelWidthToWorldScale()
	
}// CameraHelper