Mark, I charge for the full size of the room.
I explain that it actually takes longer to clean a room that is furnished than an empty room (unless you're talking just the entrance).
Many times, if the room/house is 100% empty I'll give a 10% discount. It just takes less time. Easy.
Most clients understand, every once in a blue moon they don't. In 28 years, I remember only one client (that actually said so) that declined. Judging from other things she said, I think I was better off not working for her.