Heater fan - how big a hole?!!

[jamieheasman]

I'm planning on replacing my heater fan as not only is the current one sh*t but it also appears to be causing a short in the wiring. I believe I'll have to cut a hole in the wheel arch to get to it. Does anyone know roughly how big a hole I'll need to remove it? Does anyone have any pictures of the procedure that I can see?

Cheers!