Belt Material
 

How do you alter the size of drive belting? We tried heat and that makes it crumple. Thank You

Re: Belt Material
 

What you can do to change belt sizing depend on how your drive belt is fabricated.
See http://www.mulhernbelting.com/belt_fabrications.php
If your belt uses a splice you can undo the lacing/metal fastener you can undo the connector and slice a little bit off the belt and then reproduce the splice.

If your belt is endless or uses a splicing method you can't reproduce in your shop (based on your original post this seems likely). Then you need to add a tensioning pulley to your drive train. This mean that you need to add a pulley in a position that makes the pulley tight (bonus points if you make it adjustable). There are numerous threads here on CD on how to do it.