I was playing with Alexander's SNT and finally realized what button to press to make the car boost work. Anyway, I got in a plane and held the button down and noticed that you can just keep going and going and going. Look behind you, and Los Santos is no longer in sight. I kept going straight for at least 4 mins and then finally decided to shoot straight up. Look at these images I took http://imgur.com/a/hV8Ob#0 This map is huge... You could definitely fit Liberty City in here (Would be awesome if this was possible to fly back and forth.) Try it out for yourselves!