Measure the centre to centre distance between the motor and drive pulleys in the middle of the adjustment range, double it. Then take half the circumference of the drive pulley and motor pulley (Pi times diameter divided by 2) and add them to the distance measurements. That's the length of belt you need. If you can remove the pulley from either pulley take it with you so they can get the correct belt width to the places sunnybob mentioned.
Pete