It only occurred to me recently, since introducing my friends to the game and wondering what good counties to start in are, is there a way to view county holdings or base tax before starting a game? AFAIK there's no map mode for it on the game setup screen, and I don't think mods can add map modes. My closest guess would be to start a game and check a few counties around the area you want to start in to make a more educated choice, then start a proper game.