I think the real deciding factor in size is what program you are majoring in. A lot of people on this thread that are saying to get the 15" are probably not in graphic design programs. They spend a lot of time in lecture halls sitting in small desks with little room for a laptop. They might also have to do a lot of walking around with books in a backpack. For them, a Macbook would be the way to go. For a graphic design student the situation is different. Unless you need to take a lot of general education classes that require you to walk around campus, you'll probably find yourself confined to one or two areas of the campus. For my program, it's confined to 2 buildings that are next to each other. As for space, we have larger work tables or stations. Most of the students are using 15" MBPs, but I do see a few 17s" too.
However, aside from the issue of the size of computer, you really need to be looking at what else you will be packing around for your classes. Last semester, in addition to the laptop, I had about 30 lbs of design supplies to lug around. I wound up buying a rolling case because it made it easier to stay organized. I noticed by the end of semester, most of the students had ditched their backpacks in favor of the rolling case. Anyway, if you had a 17" laptop, it would make it more difficult to find space for it and all your other stuff so that needs to be considered too. In my opinion, while I think a 17" is awesome, it's probably overkill for a graphic design student. The 15" is the sweet-spot for size and performance.