Check area population of the specific "territorial areas" of the respective teams, on page 204 of
http://bizofbaseball.com/docs/MajorLeagueRules-2008.pdf
Just to add, for the portion of Fairfield County you're looking at for the Mets and Yankees, the populations are from the cities/towns of Greenwich, Stamford, Darien, Norwalk, Westport, Fairfield, New Canaan, Wilton, Weston, Easton, Ridgefield, Redding, Danbury, and Bethel.