My room is 13.5 x 22 x 8, and is wood-panelled. I have the speakers placed along the short wall, firing along the long dimension of the room. See the pix and the last paragraph of the first post in my system description for further particulars.
Obviously there are many variables and dependencies that relate to your question, including the radiation patterns of the particular speakers. But FWIW, in my particular setup I've been very pleased with tonality and imaging, and the one significant room-related anomaly I've perceived is a deep bass suckout centered at around 45 Hz.
I've determined via measurements with test tones and an SPL meter that the cause of that is a rear wall reflection, the distance between my listening position and the rear wall being about 6 feet. Which surprised me somewhat, because the central part of the rear wall is an opening to another room.
Rear wall reflections will cause a deep bass suckout, to some degree, centered at a frequency in Hz of about 281.5 divided by the distance in feet from the listener's ears to that wall (281.5 is the speed of sound in dry air at 68 degrees F (1126 feet per second) divided by 4).
So while placement along the long wall would have the advantage of reducing the magnitude of side-wall reflections, a potential disadvantage is that the reduced distance between the listener and the rear wall will raise the frequency of that deep bass suckout. The last paragraph of Newbee's post, describing his experience, seems consistent with that. And of course additional issues may come into play with long wall placement if the speakers cannot be positioned far enough from the front wall to be optimal for their particular radiation patterns.
As is usually the case in audio, tradeoffs and compromises are inevitably involved, which are generally system and room dependent.
Regards,
-- Al
