the first thing you need to ask yourself when making a college carpeting purchase is this: how easy will this be to clean when someone spills an entire beer on it... or worse? the second thing is a natural follow-up: how much do i want to spend on something thats going to get beer spilled on it... or worse?
this means that regardless of what you purchase, it needs to be low/no-pile and inexpensive enough (this is relative, of course) that you will be willing to throw it away at the end of the year. i have a great picture somewhere of 'or worse' embedded in an expensive shag rug if anyone needs more convincing.
as for size, the bigger the better. most typical double rooms will accept an 8/9x12 rug very nicely.