ExecuteInEditMode Sample

AbsoluteScale.cs

using UnityEngine;

[ExecuteInEditMode]
public class AbsoluteScale : MonoBehaviour {

	public Vector3 scale = new Vector3(1f, 1f, 1f);

#if UNITY_EDITOR
	void Update () {
		if (Application.isPlaying)
			return;
		if (transform.parent) {
			var parentScale = transform.parent.lossyScale;
			var relativeScale = new Vector3(
				scale.x / parentScale.x,
				scale.y / parentScale.y,
				scale.z / parentScale.z
			);
			transform.localScale = relativeScale;
		} else {
			transform.localScale = scale;
		}
	}
#endif
}