Hey,
I would recommend against a 17" for the same reasons mentioned before. The footprint is too big and just isn't worth it. In all of my classes i require the laptop, even calclulus and physics. So with the laptop, subject textbook, and some loose-leaf to workout the problems there really isn't any room to spare. And i have the 15" which is "just right", a 17" would just cause problems and slow everything down.
I recommend that you go to the school you're going to, if possible, and check out the likely places you'll study in (library, study rooms, cafe...). Check out the table sizes they have. Usually libraries have the largest tables while classes have the smallest.
Whatever you go with i recommed the following: A good case, keyboard and screen protection kits (PRICELESS) and a lock of some sort to keep it safe. The screen and keyboard protection kits are pricless since people will always be touching your screen and there are always drinks (coffee) around which can cause problems if spilled on the laptop.