Speaking about a "short wall" dressing room. On a slant load trailer, most of the time it would be the side the horses head is on, most of the time the street side.... Say it it was a 2' short wall trailer, then the other/opposite/curb side of the trailer would be something like an 6' long wall.  2' short wall pictured below.

In a dressing room on a slant trailer, the wall dividing the horse is on an angle from one side to the other. This makes the side wall differeing lengths by usually 4'. The short wall is usually on the left wall and can be anywhere from zero to whatever length. The right wall will usually be 4' longer.

look at floor/ box length also - i have seen bison trailers advertised as 10'sw and they were actually = to my 12' sw 26' box.

Thanks everyone for the explanations!  I think I've got it now :-)