I just got 17's and they look great. The sidewall is quite thin, just how I like it, but there is still plenty of cushion for the rim. Any bigger and you run at a higher rise for bending your rim.
Plus, any bigger and you would have to inflate your tires more often, just to keep them nice and firm. The less sidewall you have, the more you have to take care of the tire. If you start losing pressure in one tire, you significantly increase the risk of bending your rim.
I'd stick with 17's.