How does your zoom work. Is the camera moving closer to the object as you zoom in, or do you change the field of view of the projection matrix?
If you move the camera closer, you could just multiply the objects scale by the distance between the object and the camera. So if the camera is twice the distance away, the object get's twice as big, so it should take up the same space on the screen.
If you are changing the camera's FOV, you could just multiply the object's scale by the FOV value.