I had this in Unity for a while. I ported it over to UE4
Select the dungeon actor and go to the Experimental category and enable “Fast Cell Distribution” and specify the dungeon width and length in unreal units. This ignores the NumCells parameter and generates cells (rooms and corridors) inside this [width,length] range
[Edit: This will be available in the next update 2.10.0]
https://i.imgur.com/FW0HdZc.png
https://i.imgur.com/m1FgF7Ph.jpg
This also has better performance for larger dungeons. It works with negation volumes
https://i.imgur.com/MwGMFS2h.jpg
You can create narrower dungeons too
https://i.imgur.com/LRJRPWA.png
https://i.imgur.com/u9BbRK4h.jpg