bikesdirect.com has a sizing chart for the bikes they sell. Note Bill H. and Junkman are correct that you need the right size bike. If you get a poor fit, then you will end up hating the bike. Bill G.
Approx. size would be about 54cm/21"
www.oldbikesbelong.com These guys are great. you'll have to take a drive to go get it but it's probably your best bet.
Thanks Mark! I'll check them out