I have to say, the bigger the better. Go with the biggest tank you can place beside the tank that you are comfortable using.

I say big because the bigger the sump, the more water volume you will have on your system, which is always a good thing. If you are trying to stay small as possible, make sure you have room for everything in the sump you want, and a bit of extra room for upgrades.

Other than unsightly equipment and plumbing, I have never had a problem with a sump along side of my tank instead of under it. You might want to elevate it off the floor though. It will help when its time to syphon it out for cleaning.