you don’t necessarily need to play in Sandbox to start with a bigger airport. If you just want to increase your funds a little or a lot, you can either use a money cheat or edit your funds in the GameData.json file in the saves folder of Airport CEO in your user data (let me know if you have no idea what I’m talking about).
However, if you wanna take the challenge and don’t use any cheat or change the game data, I suggest that you take the two million dollar start option, because if you take less than that it will be extremly hard to start with anything other than GA traffic and with that it takes forever to make a decent profit.
With two million dollars you can start with something that looks like this:
Later on I added a baggage claim area on the left and more check-in counters on the right. Also I left enough space between the terminal and the taxiway to build medium stands eventually. Later it looked like this:
As you can see I built that terminal on the edge of the map to leave enough space to build a new and bigger terminal within a second runway. For me that was a good way to start and left plenty of room for development.