how big the sump should be or how powerful the pump should be?
for a sump a 30g long would be perfect, or a 29g, 20gL, or if i recall the standard 40g typically used for sumps is 15" wide (or at least the company aquarium masters produces a 40g sump that's 15" wide.)
as for a return.... i'd go for something in about the 900 - 1000 gph range... unless you have alot of head pressure and then i'd spring for a little higher capacity pump. try the mag 9.5, great size for a 90g sumped system. if you're looking for an external pump then the iwakis and panworlds are great.