In my experience there is a very loose and tenuous correlation between one’s room width and the sound stage width presented by a system in that room.
I'm inclined to agree. Soundstage width is in our head and measurement - if you can call it that -- is subjective. Room width correlation is tenuous. My current room is 16ft wide and in my previous (larger) house my audio room was 26ft wide. Since it is a psycho-acoustic phenomenon it's not unusual for me to hear a soundstage extend beyond physical walls. I believe there is better correlation with other factors such as: the recording, the components.and the distance between speakers (rather than the distance betwen speakers and walls).