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