If you're talking about what's the largest tire you can put on a 13" wheel...well, that would be a 13" tire. If you're talking about how big of a wheel and tire combo you can put on your car, people have put 18" wheels on a Sentra with 205 width tires. Personally I'd go with either 15" or 16". I've got 15x7" wheels with 205/50/15 tires and love them. No rubbing or anything. I've heard of 18's with 205's and no rubbing as well, but no personal experience.