I think that as long as you can fit the necessary pipe size to handle your overflow, the box doesn't really matter...yuo can just make it to the dimensions you need. I think a 1" PVC
pipe will allow approx. 600gph, but don't quote me on that.
But anyways, I don't think the box size matters...all I know is that I read the longer the box on the interior, there is more true surface water drawn in, and less subsurface (though only by a cm or so) water. The more surface water, the more DOC
is skimmed from the surface by the overflow box. I'm cheap, so I just made the neat little overflow that BillyZ designed. Cost me $25 to build 2 of them at 1¬Ĺ" pipe. =] Hope this helps you out!
125 gallon saltwater: 55gal sump, 4 Blue-Green Chromis, Purple Tang, True Perc, Firefish, Royal Gramma, 5 Ricordea, Bubble Coral, 15 Pulsing Xenia, Green Star Polyps, Deresa clam, Green-Tip Torch Coral (about 11 "heads"!), Orange Montipora Cap, Purple M. Digitata, Green Slimer Acro. Yongei, 3 Orange M. Digitata, Pink&Green Acro. Millepora