After 10 hours in SnapMap, it seems there is no way to create a larger area or an outdoor area. There is no skybox sort of room to fill in. I had two extremely simple multiplayer maps I wanted to make that rely on this, but cannot do it via SnapMap. One solution to SnapMap could be to include a resizable open room which you could set its size and skybox in its Properties.
I tried putting multiple square rooms together but they still have a wall and door between them. There is no way to remove their shared wall/door. Maybe an option to MERGE two identical rooms together would be a workaround for this issue. As is though, SnapMaps are very cramped and the Setpieces, which are larger, are practically maps in and of themselves, leaving minimal room for creativity.