stock size belt will do fine...however, if you start to hear some squeeling...spray some wd40 on the belt...if the squeeling goes away buy a goodyear gatorback...
it wont hurt anything to give it a spray to isolate the problem...I had a belt squeal on '97Gt and sprayed it with wd40 and had no issues...I found out the STeeda pulleys had stretched the belt (the car 74K miles at that point) so I replaced it with a gatorback and never had a squeal again(sold the car with 93K miles)