StarForge and SCM Draft allow you to adjust map sizes to anything you want. Just know that you won't be able to open the map with StarEdit/SCXE. Also, for people to play the map they must play another game in that session of StarCraft before playing the map or it'll crash. I think it has something to do with problems loading some map related information from maps with irregular dimensions, so theoretically you
may be able to join and leave a game without actually playing the first one, but don't quote me on that. See if it works before doing it for real, and make sure people know they must have played a game or it'll crash. It won't crash until the game actually starts, they'll get to the briefing but StarCraft will close when the briefing finishes and the map tries to load.
12x20 is fun.
I believe 1x6 is the smallest possible working map size. Having any dimension above 256 will crash StarCraft (and StarForge, but not SCM Draft). Just know that the minimap goes to hell.