Instructions
Lift the top flap and examine the flap closest to your horse (the sweat flap). Many manufacturers will stamp the saddle's information, including tree size, into the leather. Look for letters (usually denoted by N, M, MW or W) or measurements in centimeters; this is most likely the tree's size.
Look at the billet guard (the flat piece of leather attached the long leather straps you buckle the girth to) to see if there is a stamp on it. Sometimes this is where the manufacturer places the saddle's information.
Examine the stirrup bar (the metal your stirrup leather attaches to) for an engraved number or letter. Some manufacturers place the tree's measurement there.
Contact the manufacturer if you cannot find the saddle's tree size stamped or engraved on the saddle itself (or have found the stamp but it has worn off over time), and give the saddle's serial number. The serial number is usually stamped on the sweat flap or billet guard.
How to Find a Saddle's Tree Size
It is important to know (though far from being the only consideration) the tree size of your saddle, whether you are in the process of buying a new saddle or evaluating the appropriateness of your current saddle for a new horse. After all, if your horse is wide and "mutton-withered" (where there is barely a bump between the horse's neck and back), you know immediately a saddle with a narrow tree will fit the horse horribly. However, many brands mark the saddle's tree size onto the saddle differently, so you may need to look in several areas before finding the exact size. Since saddlemakers vary widely in the terms used to describe tree sizes (even saddlemakers that describe their tree sizes in centimeters do not use a universal method of sizing), trying to "measure" the tree size will not accomplish much.