Because most Living Quarter Trailers are Slant Loads, the "Bulkhead Wall" that separates the Living Quarters from the Stock Compartment is on a slant. If you have a Slant Load, one of the Living Quarter Outside Walls is Shorter in Length than the other Outside Wall. The distance from the Front of the Trailer ( where the Jack is located ) - to where the Bulkhead Wall starts the slant, is the "Short Wall" and even though the other wall is longer, industry says the size of a Living Quarter is determined by the Length of the "Short Wall".
The Short Wall can be on the Drivers Side of the Trailer (Street Side) or the Passenger Side of the Trailer (Curb Side). More often than not the Short Wall is on the Street Side of the Trailer because most Slant Load Horse Trailers are built with the horses facing on coming traffic when loaded.
If you have a "Full Mid Tack", the bulkhead wall that separates the Living Quarters from the Mid Tack is Straight across the trailer, therefore each Outside Living Quarter Wall measures the same, and the Size of the Living Quarters is determined by the length of either Outside Wall. The Bulkhead Wall that separates the "Full Mid Tack" from the Stock Compartment is normally on a Slant. (see more in FAQ about "Short Wall and a Full Mid Tack".)
If you have a "Half Mid Tack", this area is considered part of your Living Quarters when it comes to measuring and determining your "Short Wall" Length. (see more in FAQ about "Short Wall and a Half Mid Tack/Mud Room".)
See also in this section of FAQ - helpful prints about "How Do I Measure my Trailer?" Height, Width and Length.