Unless your RA knows about fish, they really probably wont be able to guess the size, ours didnt. We were allowed a 5 gallon, and ended up with two tens. A betta works well, and is quite a showpiece if you raise it and make its finnage large. It even became a trend and at least 5 ppl on my floor alone had bettas.
Anyhow im now off campus so my roomate and i have the following: 90, (2) 55's, 20, 12 eclipse, (2) 10s... Will probably get more tanks soon, and i have another 55 sitting at home but im kinda running out of room for living though, and my parents keep telling me im there for college, not fishkeeping.