One factor you need to keep in mind, even with small games of naval/space, is that quite often the ships have very large turning circles. You must make sure there is enough room in either direction for them to turn around, or you will do the first pass with each other then all nick off the other side of the table, which is rather awkward. (Yes, I did it with my first game of Battlefleet Gothic!)
Dystopian Wars isn't too bad for that though, most ships can pull a 90 degree turn with only 3-5 inches extra forward. I'm personally sticking with a fairly standard 6x4 foot table, my pool table.
4x4 would also work for smaller games, but wouldn't scale up all that well.
As to islands.. Really up to you. Also depends on how many land units you want hanging around to start with. If you are sticking with sea and air, just scatter a few smaller islands around the place to break it up a bit. I think when I start getting a few people familiar with the game, it would be really cool to have ~1/3 of the table a beach/landmass and have the ships able to do shoreline bombing on land units, and them able to shoot out to sea.
