when opening a base, there is a listbox with base size(I don't remember the exact name, I already have the maximum). Be sure to check it. IIRC, the price for a level 4 base is nearly the double than for a level 3. If it pushes you into debt, it can be a killer.
Anyways, don't open a second base too early, it's looong to make profit, as it's only new lines, and those can be deadly. The usual wisdom is to begin by short range routes, with few opposition, and a lot of demend, to insure the first planes are quickly making profit(hint : Tahiti is not a good base for that strategy).