I spent 6 months at Waseda, albeit in a homestay. My experience regarding the dorms is somewhat mixed, as they pretty much prohibit visitors. The rooms are small, but no smaller than dorm rooms in any American college campus (probably somewhere around 12 x 14, ifrc). Some of the dorms my friends stayed in were up to an hour away from the actual campus, so it can be somewhat of a crapshoot when applying. If they give you an option, I'd certainly opt for something within walking distance, if only to save on travel costs. The rooms I snuck into, whether to hang out or pass out after a night of drinking, all had windows, were clean, but used communal bathrooms.
Waseda was a fun time. The international building is always full of energy, located near plenty of good food, and packed with people looking to make friends, both foreign and local. The rooms Glenski mentioned, with the chalkboards, are much older than the international building, but we still had class in there once or twice a week. The Japanese classes were especially intense in the beginning, but as the program wears on it seems like interest wanes a bit, both on the part of the students and the teaching staff. The other classes are kind of a toss up. I had extremely interesting, involved classes regarding law, government and culture, but the business stuff was as dull as can be.
My advice for ANY prospective study abroad student is to push yourself out of your comfort zone: join a club, talk to someone you might not talk to, invite people to go on random adventures. Some of the best friends I made were people I thought I'd want to avoid in the first couple of weeks. And have fun!