Space and Underwater maps should be medium at least. Having them small wouldn't leave much room to be able to maneuver around.
I'd have the Space Map be the ODIN / LOKI space station. You'd be able to travel through the station itself down narrow corridors and such, which can be good for those who prefer more CQC, or you can hang out outside and use debris for cover for more medium to longer range combat, and you can interact directly with doors to open and close them, thus changing the layout of the station slightly, or to help escape pursuers easier and such.
A Field Order could be self-destructing the station that would instantly kill anyone inside it or, if you're close to it, send you sprawling around, vision distorted, and damaged slightly, while also hurling out more chunks of debris, thus more or less changing the layout of the map. A fun environmental hazard could be that if you go too far down towards the Earth, you automatically die by burning up (which you could also end up flung down if you're below the station if it's self-destructed). A bunch of Streaks would also be unusable or less effective given, obviously, you're in space.
The Underwater Map would have 2-3 sharks as an environmental hazard that will swim pretty much all over the map (and they can't be killed, so it'll add more need for being aware of your surroundings and also discourage camping a bit for those who hate it so much), and they'll be more attracted to wounded players (2-3 bites will kill you at full health). Similar to the campaign, there will be underwater ruins people can fight CQC in or take cover in. Like in space, a bunch of Streaks would be unusable or less effective.
A Field Order could call in a submarine that will make a couple passes through the map and emit the sonic sonar pulses from the campaign, and enemies can avoid taking damage (2 hits from them would kill) by hiding behind cover (which will be destroyed by the pulse, which can alter the map slightly).