The size sump you use depends on how much room you have under your stand. In my opinion, you want to use the largest tank possible, that'll fit...still leaving you room for equipment (skimmer, etc). I have a 45gal sump, for my 90gal tank...and that's only because I cant cram in a 55gal+.
Check out
www.melevsreef.com and search thru his "Acrylics Works & DIY" section. He has everything from basic, 3 baffle sumps, to very complicated, multiple chamber sump systems.