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.

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.