Hmm. I would assume that it is probably either only the square they are on, or next to. Leedot made a post regarding the difficulty of holding particular planets, especially those Gas Giants. Because you can't colonize a gas giant, you can't use orbital turrets, which makes them harder to hold. In my opinion, if you had a colonized world in the same system, if all ships/orbitals attacked in the same system, would be very unrealistic.