The terms used in saddle tree sizing are explained in Richard Sherer's book on western saddles, and they deal with the distance between the bars, measured at the gullet. These widths may not be consistent between different saddle tree makers, but it gives you a starting place. The width of your horse's shoulder determines the type of tree and gullet width you will need.
Regular quarter horse bars - gullet width: 5 3/4"
Semi-quarter horse bars - gullet width: 6"
Full-quarter horse bars - gullet width: 6 1/4"-6 1/2"
Extra-wide quarter horse bars - gullet width: 6 3/4"-7"
Arabian bars - gullet width: 6 1/4"-6 3/4" (has a flatter pitch than quarter horse bars)
Source: Sherer, Richard L. Horseman's Handbook of Western Saddles. Sherer Custom Saddles, 1988.
