Dear Jeremy,
If you have the original its quite easy, look up on google to you nearest belt distributor, there are many in most towns, they will be able to measure it correctly, failing that, if you have not got a belt, you need to measure, the length around the pulley wheels, I find it best using washing line. also measure the internal width of the pulleys, from there take the measurements to the same distributor
When you have done that, I always buy a spare, for the price its not worth the heart ache of another breakdown
Regards
Andrew