How to get Game object size and fade it.

Use a shader, and change the properties.

gameobject.transform.localscale

local scale is a Vector3, so

 IEnumerator FadeGameObject(gameObject go){
     float speed = 0.1f;
     while(go.transform.localscale.x > 0.01){
         go.transform.localscale = new Vector3(go.transform.localscale.x - speed, go.transform.localscale.y - speed, go.transform.localscale.z - speed);
         yield return null;
     }
     // then turn the game object off
     go.SetActive(false); 
     // OR destroy the game object, removing it from the scene completely.
     Destroy(go); 
 }

To activate an IEnumerator use

 StartCoroutine(FadeGameObject(myGameObject));