Long wall is best for many of the reasons above, especially the sidewall first reflection issue Tom mentions.
HOWEVER, the short dimension of the room must be 15 feet, bare minimum: 2.5 ft minimum behind the speakers which are 7 ft apart (midranges), + 1.5 ft speaker cabinet depth (average) + 7 ft from the front of the drivers to your ear (when seated) + 1 ft chair or sofa back, + 3 ft for bookshelf (abfussor) and walkway behind sofa. Total = 15 ft, and the long dimension must be 21 feet or more.
You can try and eliminate the 3 ft bookshelf/walkway, but even with absorption on the rear wall (behind the sofa) if you shove the sofa against the rear wall, the bass will be pretty boomy and you won't hear a good soundstage. So I'd recommend you go for the short wall. 13 ft is a decent width (12 ft or less is a bitch!) and you will need sidewall treatment to kill the first reflections. You can pull the speakers out 5 to 7 ft from the rear wall (a very good thing) and put your ears about 7 ft back from the drivers, leaving you 6 to 7 ft behind your (ears) listening position.
The reason for 21 ft being the minimum long dimension, is that with speakers centered on the long wall about 7 ft apart, the path length for the first reflection from either speaker to the sidewall and back to your ear is approximately 17 feet. Since sound travels approx. 1100 ft/sec, it takes the reflected wave about 15 milliseconds to reach your ear while the direct sound, travelling 7 ft, takes only 6 milliseconds, a difference of 9 milliseconds. As long as the difference is at least 7 milliseconds (if my memory still works ;--) your brain will hear the reflected wave as an echo (which is OK) but if the difference is less than 7 milliseconds, your brain will combine the direct and reflected sound as if it was ALL DIRECT SOUND and that's what SCREWS UP THE IMAGE AND SOUNDSTAGE.
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