Lecture hall desks are usually on the small side. Get a 13 and an external monitor so you can do some real multi-tasking when you're in your apartment / dorm.
Thanks to everyone for the advice. I don't do anything intensive besides some very light Photoshop so I'll go with a 13 inch. Still deciding whether 8 GB to 16 GB upgrade is worth $180
Desk or table you should be fine. A seat on a airplane, I found the 15 inch to be too big. With the tray down it was hard to open it far enough to have a good viewing angle. My 13 MacBook Air was perfect for flying. Dorm or classroom should be fine fir a 15.