OnMouseEnter over a Collider, what's the distance?

I'm trying to use OnMouseEnter (and Exit) on some colliders to be able to enable some GUI elements.

It works fine, when I'm within a certain distance (it seems). I don't see any way to tell Unity what that distance is (like you can with raycasting). I would think that no matter what the distance, if the mouse is over the rendered collider, it should get these mouse events.

I'm using a mesh collider which is a mashup of two cubes, one facing 'out' and one facing 'in', so that even if you are inside the cube collider, it should still detect the 'inside' faces and register a 'hit' (mouse is over it), right?

The default raycast distance, which OnMouse events use, is Mathf.Infinity. i.e., there is no distance limit.