For an average sound room, you don't want any dimension smaller than 15' or so, with the other dimension being at least a couple feet longer. You want to avoid square rooms.
So 15' x 18' would be pretty good for a small pair of speakers, 25' x 19' or something would be better for a larger pair of speakers.
So 15' x 18' would be pretty good for a small pair of speakers, 25' x 19' or something would be better for a larger pair of speakers.