If you look at the sticky one thread above this one, you'll find your info. I've got 215/40/17's on my car. In my opinion it looks perfect. The width really fills out the wheel well and is almost flush with the body of the car. I debated getting some 18's on there, but the tire place talked me out of it. They told me it'd be about $500 more!
Actually they just told me that I may notice a decline in the ride quality and that they are easier to blow out than 17's. I really like the look of 18's on these rides though. If you can manage to not get rims without a lip they ultimately look bigger than they are. People mistake my 17's for 18's quite frequently. I think it just ends up depending on what you want out of your tires and rims. If you are a big commuter, I wouldn't recommend anything bigger than 17's... if not, then go for it!!