Wait until you've moved into buy the rug. Even if you're school gives approximate dimensions for your dorm room, who knows how you'll want to set the room up, etc.
Measure the room. College dorm rugs don't have to be expensive if you buy only the amount of rugs you need. Cover any high traffic areas and anywhere with nasty stains to make the room look better.
Talk to your roommate about his or her taste in decor. Decide together what to look for in your new dorm rug. Remember dorms are cramped and often messy, stay away from anything that white or that will otherwise get trashed easily.
Shop online and in the store. Rugs and carpets are expensive. By shopping around and comparison shopping, the best deal can be found. Often around go back to school time, dorm sized rugs are on sale at big box retailers. Rugs don't have to be expensive. I found some reasonable rugs at Amazon for under $50. I've included them in the resources section below.
Buy your rug, take back to the dorm, and enjoy!