Hi Chris,
There are no universal codes for belt size, the suppliers order codes can give an indication sometimes!
Most are diameter size, turntable belts are sometimes the circumference.
CPC have a good selection, all diameter. Wiltsgrove and VAS also have a good range, again listed by diameter, but they only supply to the trade. Don't be tempted by cheap ebay Chinese stuff, you will waste your time and money as they don't last very long!
Keith