I purchased this about 2 months ago for some rollers that are about 20 years old and it seem to work fine. The measurement is the length(or circumference) of the belt. If you have 4" drums, measure the spindle to spindle length and add 4". If it's close you should be fine. Also just check that the diameter of the cord will work with the grooves in your drums. I don't think that it's anything too critical though.

http://www.bikeman.com/WT1006.html