my stereo room is ~ 20' x 12'. I've lived in several different houses, and tried many rooms, this one is the best acoustically that I've had. You'll want that extra 3 ft so your chair doesn't need to be against the wall, and you have room to pull the speakers out into the room as Jafox says. Mine are 3 ft out from the wall, and my chair is 3 ft in from the back wall - which is what works best for me.
Audio room dimensions
I have and area in my basement where I'm going to build my audio room and have a couple of options. The easiest by far is to build the room to 12'9" by about 16' perhaps a few inches less on the 16. Or, 12'9" by about 19'. The latter will require I do some serious surgery to the area at the bottom of the stairs where the door at the bottom of the basement is. My question is: will the smaller room seriously compromise the sound vs. the larger room? The finished ceiling height will be about 7'. Thanks.