How do you measure gullet size on a saddle?

Unfortunately, no standards for gullet measurements exist within the saddle industry. Most gullet measurements are taken on a raw saddle tree and not on a finished saddle. Once a saddle is finished, there is really no way to determine the gullet width.
A good rule of thumb to use for gullet fit is that there should be clearance of two to four finger-widths between the withers and the gullet.
Standard measurements for gullets described in Horseman’s Handbook of Western Saddles are:
Regular quarter horse bars: 5 3/4″
Semi-quarter horse bars: 6″
Full quarter horse bars: 6 1/4″ to 6 1/2″
Extra-wide quarter horse bars: 6 3/4″ to 7″
Arabian bars: 6 1/4″ to 6 3/4″ (has a flatter pitch than quarter horse bars)
The gullet is the tunnel underneath the fork and rides over the horse’s withers. The design of the fork and the angle of the bars of the saddle tree determine the width and height of the gullet.