I explained this on the board once before, but here goes again.. the easiest way to size a new belt (provided you are going smaller) is to use the old belt, put it on, and use a pair of needle nose vise grips to take up a loop of the slack on top... that way you can check the tensioner and make sure that you've got enough movement left when the new belt stretches etc... once you've found a good length, pull the old belt with the vise grips still attached and then go to the parts store. Keep checking against what you've got until you have a good match... this method has worked flawlessly every time for me.
-Jim A.