You generally would want to start out in the game world in an airport that is capable of supporting a very high level of demand. That's the best strategic choice since you can't move your HQ (.../...)
It also depends on your objectives. If you want to conquer the world, Heathrow is a good starting point...but you'd better be better than others to survive here. If you want a more quieter game, you start at a smaller place, acknowledging that you might be limited in the game.
But anyone feeling limited by the 600 airplanes rule should give a try to a big starting airport. We don't speak about newbish amateurs, there.