Either the long or short wall can give you good speaker separation. But in the case of the short wall, make sure side walls are not very close to the speaker, as this disrupts image localization. Correct speaker separation (for any given chair location) requires a very simple process of starting with speakers close together, listening to music, and incrementally widening the distance between them until a hole in the center of the image occurs. As long as the speakers are too close together, voices or other central sounds will sound like one source coming from the center. As soon as the distance becomes too great, each central sound eminates from two positions (to the right and left of center) instead of a central position. So, when you hear two identical singers, your separation is too great. Move the speakers back to the exact position a central singer snaps back into focus, and you've got the widest separation possible without a hole in the middle. If chair location is easier to change than speaker separation, then the same thing can be accomplished by leaving speakers where they are, but moving the chair closer until the hole appears. Then move the chair back until the hole just fills.