Once I've determined, qualitatively, which markets might be most appropriate for my web application I try to calculate the maximum possible size using data from the U.S. Census Bureau (for US markets obviously.)
The 2007 Economic Census gives a pretty strong industry by industry breakdown and it's good for coming up with a "maximum possible market size by industry / sectory / region" estimate.
The 2007 Economic Census gives a pretty strong industry by industry breakdown and it's good for coming up with a "maximum possible market size by industry / sectory / region" estimate.
http://www.census.gov/econ/census07/
The Economic Census is astonishingly comprehensive, and it's freely available - well worth the look.