Here's the MLB blackout map regarding particular markets and the ones that get blacked out for home games. If nothing else, it can give you a visual of where markets are, etc.
http://upload.wikimedia.org/wikipedi...lackoutMap.jpg
Another thing to consider about market sizes is, whether or not your league has revenue sharing. For example, without revenue sharing, it's extremely unlikely that the NFL would have teams in places like Green Bay, Jacksonville and Nashville, but not Los Angeles.
If you don't have revenue sharing, there are always going to be small market teams and as a result, teams that aren't doing well. The game does an extremely haphazard job of figuring out what markets are "large" and "small" because the game doesn't base it on population, so if you sim for too long, it'll change a team's market size based on success, which has very little to do with it for a team in a metropolitan area (E.G. The yankees)
So...it's a philosophical thing, almost.