Long or short wall

There are so many variable right down to what speakers you have, that trial and error is your best bet.

That said, I would probably default to putting the speaker against the short wall if I only wanted to do it once.

It might be easier if you gave the demensions of your room.From the VSA pic,I would say that the speaker is too close to the side-wall(3 feet minimum).But if putting them on the long-wall doesn't give you 7.5-8 feet from the front-plane of the speaker,you won't have enough distance and will cause other problems.

Sugarbrie is right about defaulting to the short wall. I think the biggest mistake the general public makes about speaker placement is that of treating sound projection like HVAC. You're not pumping air into a room, so the popular notion of "dispersing" sound into all areas equally doesn't apply. Not only does sound travel way faster than blown air, but our experience thereof is more about direction of its origin, than feeling it around us like a vapor.

The other day, I was in a restaurant where they had a zillion little fancy speakers suspended down from the ceiling, like they were trying to distribute the music intimately to every 100 sq. ft. of the dining area. It totally sounded like ass. Would have been infinitely better to have a couple of larger speakers at one end of the space, at ceiling level but angled just slightly down from horizontal.

You may have to look past the room depending where in the house it is. My system is on a far short wall. The other end is wide open into another room, except for the typical slight left/right/ceiling divider.

While the system room is almost square (18X16), the length is actually more than double because the far wall is at the other end of the adjoining room. It is practically one large long rectangular room as far as the sound goes; not the two smaller rooms.

The sound fills both rooms (which if fine by me). If I put the speakers on the 18' side, I would have a close solid wall on one side, and an open side into the other room on the other side. It would create all kinds of problems put that way.

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.