Change UI scale in relation to 3D camera

I have an nGUI sprite that I use to point to different parts of a 3D object. I would like the sprite to scale so that it remains consistent when the player zooms in/out from the object.

What is the best method for this?

I already have a method that records the Camera.main.transform.position.magnitude but I'm not sure what to with it from there?

Thanks for any help!