When is a car considered to be "fast"?
When is a woman considered "busty".
The answers to the above questions would be as varied as to the answers regarding room size.
One thing to remember, the size of the room is not just the square footage, but the also the cubic volume of the room (LxWxH).
On the B&W and Axiom websites there is a speaker suggesting software that recommends speakers based upon room size.
My thoughts regarding room size:
(assuming 8 ft ceilings, solid construction, self contained room with closing doorways)
Small Any room under 2000 cu ft (approximately 15x18x8)
Medium Any room from 2000 to 5000 cu ft (approximately 15x18x8 to approximately 20x30x8 or 18x28x10)
Large: Any room greater than 5000 cu ft.
Very Large: any room greater than 10000 cu ft (24x40x10)
Obviously, each of us will have a difference of opinion as to what small-medium-large really is considered.
I suspect speaker companies would be the same way. More importantly, British companies, due to the typically small home size in the UK, would be more inclined to rate the room size much different than an US based company.
PS: Don't forget, the accoustics of the room are just as important.
Big, heavy, overstuff furniture in a room with a thick carpet and pad = a dead room.
versus
No drapes, a wall of glass windows, no carpet, wood furniture = a lively room.
The dead room will require more power to generate the same sound volume as the lively room.
