Depends on the car (because the assembly tolerances are all over the place on these cars), how low (as in how much of the tire you want up in the wheel well) you want it, and if you are willing to fold or cut the fender lips. On mine I can fit a 305 45 17 (almost 28" tall) with a rolled fender lip, but the tire will hit the inside of the wheel house both inner and outer sides if it's too low or you hit a really big bump. Goody has 295 18's on his and I think they clear everything. If you go with a lower profile tire, you can fit more width because the tire is shorter.
Spend time measuring your car - ymmv