How to define a room with pre-built structures?

[Obsidian]

Some odd mechanics go into defining rooms, I'm constantly having problems with rooms being designated bigger, or not at all relative to the structures I build or repair from shells pre-existing.

One mod I'm using adds a bunch of additional structures to map gen, but I've had problems with them, in one case, it's 6 buildings, 4 7x5 rooms and a few additional ones. The 7x5 rooms where completly incased in a mountain, but accessible enough via corridors. I add some vents, doors and check the roof, but all 4, seperated by significant corridor space, plus the whole corridor area, are defined as a single room for sharing purposes.

I've had additional trouble getting the game to recognize structures as being built for rooms and the roofing AI breaks frequently...

Any manual mechanism for defining the dimensions of a rooom?