The dorms at the University I attended each had a drawer unit with a large clean top that I was able to place the 20 gallon on. People loved coming in to the see the fish, but I have to admit, I was learning and I let a lot of great fish die with that tank! I still remember the tacky no fishing sign I put in it...
those were the days!
I recommend a 20 too because the larger the better in terms of maintaining good water quality, but at the same time, you don't want something too big for college, especially if you go home for the summer and breaks, someone has to care for the tank while you're away! A 20 makes it easy to manage the transport of the water (you should always save at least half when you move), and the weight of the tank!