No one is going to be able to answer this because every setup is different, you are just going to have to use a string technique, keep cutting the string until it wraps perfectly tight, measure that then take off like 1" or so for tensioner (there is no exact number here because it depends on the angle of the tensioner). You will probably buy 1 or 2 of the wrong sizes before getting it right so this is best to get locally so you can return.
Belts have the size as part of the part number, for example the belt I needed had to be 81.5" , So I used a Gates k060815 (06 ribs, 0815 length), which luckily was in stock at the local auto part store.
86 Fiero GT 4spd - L67 swap: VS cam, GenV. Needs exhaust (ETA 9/13) Build Progress
98 GTP - Some mods