I'm spreading the power among ceiling, floors, and walls.

A 20 foot square room (10 feet tall) would have 800 square feet of walls and 800 square feet of ceiling/floor (I assumed no window.) Total: 1600.

A 2 foot square room (1 foot tall) would have 8 square feet of walls and 8 square feet of ceiling/floor. Total: 16.

1/100 of the area, so I assumed 1/100 of the total power. (Ignoring lamp efficiency, etc.)