I am assuming there is some kind of tension adjustment. If so, move the tension adjustment to somewhere in the middle. Now measure from outside to outside of the pullies, on center. This should give you a usable length. Next measure across the inside of the pulley where the belt sits, this should give you a usable width. If the width is to narrow the belt will have a tendency to work into the pulley and grab as it turns, this is evident by turning the pulley by hand with the belt installed. If it is grabbing you will see the belt move downward into the pulley as it releases during the revolution, This could cause vibration.
Best of luck to you