For those of you that want to have small islands a couple of little hints to do with map heights

Make sure the pixels are all on coordinates that aren't odd, odd starting with 0,0 in any corner. That way it makes sure no one can land on them.

If you want to make them larger and include an odd, odd pixel then make the ground types impassible eg low mountain.

This is just to ensure the AI doesn't get confused. So the islands are for show (at the campaign map level) but don't interfere.