18's will physically fit the car, for that matter so will 19's if that is the look you want. You have to stay as close as possible to the outside diameter of the stock wheel and tire package (24.88"). The profile of your tires becomes smaller and you are more likely to damage the wheel and tire when you hit something in the road.

I am running 17" rims with 225/45/ZR17's, that are only slightly larger in diameter (.09") than the stock combo, and they came out to the same weight as the stock combo.