I think even on Altis, 5k would be a bit much; a larger map than Altis would def be feasible. Extending to 3K would probably work. I agree that Air shouldn't be any farther than 2K due to the speed. Cooldown would be good, but as you said, I agree it would be hard to enforce.
If Australia is going to be a new map, and I heard its much bigger than Altis, 5K would probably be good.