Don't know if this is the photo that you have, Chuck, but if it is I make the largest of the lower pullies 1.25 times the diameter of the smallest upper pulley. So, if your smallest upper pulley diameter was to measure "N" inches multiply "N" x 1.25 and that should be pretty close to the diameter of the largest of the lower pullies.

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It looks to me like the belt length stays fixed for all four pulley steps (ie: the pulley centre to centre spacing stays fixed) so the other unknown diameters on the lower can be figured based off the belt circumference needed for the outside pair. Gates Belts has a nifty online or phone app calculator to make that part easy if you know two of the three variables (pulley A, pulley B, and centre to centre distance).

Hope that gives you something to play with. Of course the best would be if somebody happened to have the pulley sizes!

-frank