16 X as-wide-as-you-can-fit. Most track cars use 16" as the the largest type of rim. Just get as light as possible, realistically. Lower that momentum! Make sure it doesnt scrub though.
More important than the rim, is the rubber. Given that that's the part that's in contact with the road... Wide tyres are great. I think one of the guys (lgee?) managed to squeeze 235's under there... that's a friggen wide tyre! Even 225 will be plenty of meat to plant your power. Stock engine, correct?