I want to the scale of GameObject looks like always be Vector3(1,1,1) on Screen whatever the camera position be moved. I think function named ScreenToWorldScale like Camera.ScreenToWorldPoint should do it!
It sounds like you really should make the GameObject a child of the camera so that it moves with it - are you trying to make a HUD?
The only other way would be to work out the scale in a script based on the Vector3.Distance between the camera and the object.