You can manually change the resolution by editing settings.json in %appdata%\Game Labs\Ultimate Admiral Dreadnoughts, though from an examination of mine it just shows "resolution: 4". My guess is that you'll want to set it to 1, to make it as small as possible.
I've also found that it only detects resolution options at all in fullscreen mode, and only for the monitor the game last started on; if you have 2+ displays, you'll need to move it to the correct monitor in windowed mode, change it back to fullscreen, restart, and then change your resolution.