Mark covered it. Sew-Classic has those same instructions in print.
As for your OSMG he simply cannot pull a belt size out of the air. There is literally hundreds of brands of machines and far more sizes of hand wheels and motor pulleys. I asked my LSMG and he told me there is no concise belt listing for sewing machines like there is for cars.
Take the machine to him and have him fit it with a proper belt. Then record the belt number and the next time you'll have it.
Joe