I would purchase the largest one, if you are a college student. College is so busy and stressful, therefore having a nice, large agenda may help you feel like you have your life in order, even if you do not.
I have what is considered the "Large" agenda. It really isn't that large, it is about 1/2 the size of a piece of paper. I could not be happier with this purchase. Although I bought it halfway through the year, it was my gift to myself for taking summer classes. I have EVERYTHING in my planner, so the large is perfect for me.