change size of orthographic camera

If you want to use a reference variable, you should declare it as Camera and reassign the gameobject. That way you directly have a reference to the Camera component:. Also variable names should start with a lower character.

Next thing is that the camera doesn't have a variable / property that is called "size", it's called orthographicSize.

 var zoom = 100;
 var mainCamera : Camera; // assign in the inspector
 function ZoomCamera()
 { 
     mainCamera.orthographicSize = zoom;   
 }

If there is only one camera tagged "MainCamera" you could simply use `Camera.main`

 var zoom = 100;
 function ZoomCamera()
 { 
     Camera.main.orthographicSize= zoom;   
 }

If you really want to use a GameObject reference in combination with GetComponent (this is the slowest way), do it like this:

 var zoom = 100;
 var mainCamera : GameObject; // assign in the inspector
 function ZoomCamera()
 { 
     mainCamera.GetComponent(Camera).orthographicSize = zoom;   
 }