I recently dug up an old JC Penney MCS 2230 integrated stereo, and I'd like to get the turntable going again. (Despite the source, it's a pretty nice stereo system...the radio tuner works well, it seems to have about a 40Wx2 output power rating (which seems reasonable, having looked at the internals), the cassette deck is a partial logic design with the ability to skip ahead 1, 2 or 3 songs and the turntable itself is a linear tracking type.)
Anyway, before I stored it, the turntable was in need of a new belt. I set the belt aside and now it seems to be gone. I could have sworn that I read about how to determine an unknown belt size in the FAQ, but I could not find it just now. As best I remember, a string was used and placed around all the components driven by the belt, after which a measurement was taken.