There are a number of online calculators. Take measurements, plug them into the web site, and they'll return frame size suggestions -- not just seat tube length, but other important measurements like top tube length which some people might argue is even more important than the seat tube length. (Most bike manufacturers list these frame geometry specs on their web page.)
I tried the fit calculator at Competitive Cyclist's web site and its measurements seemed pretty good to me:
http://www.competitivecyclist.com/za...LCULATOR_INTRO