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.