I used to play on Kel'thuzad alliance if you don't mind a PvP server (although with the population imbalance, it's rare you'll ever see horde). It has a pretty decent AH.http://www.wowprogress.com/realms/rank/us
You can use that website to check out realm activity. It only tracks people that raid though so might not be indicative of overall population, but it should give you a good estimate. You can sort it using any of the columns.
Oh but, that site doesn't track server mergers so you'll have to think about those too.