I guess map sizes of 1000 x 1000 km shouldn't be a problem, because most of the area will be water. If we see any performance issues in the game, I doubt it will be caused by map size.
I just hope Combat Pilot doesn't make the same mistake as Il-2 Pacific Fighter. Il-2 PF maps like Tarawa or Okinawa did not have enough ocean area around the islands, so carrier task forces had to be placed much closer to the islands than in reality.