How big is it?

I read a lot about "big rooms" and "small rooms" when it comes to speakers, and they say room size matters, but how do they measure? My listening room is 21' x 24' and in my neck of suburbia it's a very large room, but they guy on the hill might consider it a closet. So when we're reading about a speaker needing a large room or small room, just what do they mean?