on a standard rim, 165/80R15 (front) and 185/65R15 rear
on aftermarket rims with no mods to the steering, fenders and suspension 165/65R15 front and 195/65R15 rear
on modified fenders, steering and suspension, the sky is the limit,.. but this is utterly pointless as there are no benefits for going any larger than 165/80R15 front 195/65R15 rear (infact it'll ruin the handling and hte acceleration)