I just recently switched from a DIY PVC overflow, and I was using 3/4" PVC on my 55 gal with a mag 9 pump turned pretty much all the way down. Would guess I was getting about 3-500gph with the 3/4" PVC. So depends how much flow you want in there. Good luck, I was always nervous about the PVC overflow losing syphon so I just purchased a CPR overflow.
-Owen