What size is the stock rear sprocket on a '96 GS500e?

39 on my '02.

pretty sure it's the same for the earlier bikes.

Yup, 39, it's also on my "mini-faq" on my homepage (Link in sig) and on http://www.mawonline.com/XGS500E.HTM

Check my site for all the possible gearings, front and rear combinations:
http://www.bikepower.net/gs500e-power/gs500e-power.htm
Go to: "the bike" and then "gearing tables". By the way, I have 39 on the rear.
Good luck!