i took a toshiba x200 class everyday for two years (it's bigger than a 17 mbp). and i plan to take my 17" to class every day for the next two years. i've never found a seat i didn't fit into, because there aren't many lecture theatre seats i've come across that aren't 15-16 inches wide.
as for the weight, get yourself a knapsack that's designed for 17" lappy's, and it's no different than carrying another textbook. i vastly preffered the 17 for the increased screen real estate while i watched movies and did assignments. a lot of my floormates used to borrow my computer to watch movies with while in the dorm, also, it's better for gaming and other things of that nature if you're not getting a console. <--- you gotta think about that stuff if you're gonna be staying in res.
although i very rarely see another person with a 17", everyone always says they wish they had one like mine when they see me watching movies and what not on campus.