I definatly would try to get the biggest sump possible. I have a 55 gallon with a 20 gal long sump and wish I had gone with a 40 gallon sump. Obviously the more water you have in your system the better, but also having a larger sump gives you more room to play with. For example, I use an octopus nw 110 skimmer and when I built my sump, I build my skimmer section just big enough for the footprint of the skimmer in order to maximize the size of the fuge. Now if my skimmer kicks it, I only have a few options to either buy the same exact skimmer or one of a similar size, or to redo the baffles in my sump. The bigger the fuge you have the better as well. If youre still in the process of building your stand and sump, definitely consider trying to get the biggest sump possible under your stand. Also dont forget about height. When I built my stand I didn't build it high enough so I was also limited to what skimmer I could get in order to be able to get the collection cup out. One more thing, keep in mind that you may want to add an ATO so you may want to have enough room under your stand to hold a reservoir. HTH
