I just used BoundingBox node and measured between min&max points. This has a faulty that it can't give correct size when the room is not aligned with world axis. So I created the next version.
This dyn measures lengths of first two curves of each perimeter of room plan. So this can show correct size even if the room is not aligned with world axis. And it can report whether the room is rectangular or not.
You can download dyn here as usual.