The best way to do it is to go to autozone or o'reilly and try a few untill you get the right size, i had to try several and finally ended up with like a 72 inch belt, but i don't have a/c either. If you can't drive it down there maybe tow it or put everything back together and redo it in the parking lot .
Another way is to take a piece of string and route it around all of your pulleys in the correct way and cut the string the right size and take it with you for reference.
One easy way to measure it is with a
cloth dressmakers tape. They're flat
and flexible, just thread it around your
pulleys like you would a belt, and
read the measurement. Anybody that sews
usually has one, or you can get one cheap at
any store that has fabrics like WalMart
or a fabric store.
i have under drive crank an water pump pullies, a/c, p/s, smog delete an used an 84 an the tensioner is about maxed out...they didnt have half sizes where i went...i'm goin with an 83 next time.