I think it might be an error to presume how speakers will sound based on just the "size of the room"; I think it's the dimensions (ie, the relationship (the ratios) between length, width, and height) of the room (plus the the shape of the room - if it's not all 90 degree angles) plus the surfaces/finishes (which absorb or reflect) that dictate the sound more than just the size (small, medium, large, etc.); and of course the quality of the sound will also depend on where/how the particular speakers are placed in the room, and also the location of the listening position. Some of these things can be easily fixed (by moving the speakers or the listening postion), or somewhat easily fixed (by adding treatments to the surfaces), but changing the dimension relationships and the angles of the surfaces relative to one another is generally difficult/expensive. In the end, you either have to be very good with calculations or you need to listen to the system in the room to really know what you are going to get.