205/60/15 and 225/55/15 are the two recomended sizes. They're almost the exact same outside diameter. If you don't mind the speedo being off my 1 mph you could get some 225/50/15. I'd recomend going to tirerack.com and seeing your options and avoid going to a 225/60/15 because that's going to give you a larger sidewall. If you want a cushier ride you could go that route though.
Kevin (President TBEA)
"WAAAAAAARRRRRIORS, come out to plaaaaay."