I think the most versatile tanks are 30 long, 40 breeder, 75 standard, 120 fat boy, 125 standard, 150 and 180. If I were to pick one of the tanks it would be the 40 breeder due to the dimensions.
considering what site we are on anything less than than 8x3x2 is redundant.
room for a couple of 2 footers or a few 12inchers.
if you want to only keep fish in the 8in to 12in range 4x2x2up is ok but its not monster keeping is it.
if you are talking aboutstandard community then 50+ is great but thats for another site isnt it?
a 75g will let you keep the vast majority of commonly available fish, if you're in a dorm room and want something exotic there's no real need to go over 20G IMO.. there are plenty of small fish, or relatively inactive fish that are every bit as interesting as the monsters most of the people here are into.