To fit a Western saddle to a horse, first place the saddle on the horse's back, on top of an inch-thick saddle pad. Tighten the cinch until it is snug, but ensure it is still comfortable. Next, the rider sits in the saddle with his feet in the stirrups. At least three fingers should fit between the pommel's arch and the horse's withers. Finally, look at the saddletree's width, and compare it to the shape of the horse's back.
If the horse has lower withers and a wide back, the saddletree must be wide. On a horse with higher withers and a narrow back the tree should not be too wide. To determine the tree's width, with a flat hand put your fingers sideways between the top of the horse's shoulder and the saddle. If you cannot squeeze your fingers into this space, the tree is too wide for the horse. A tree is too narrow if you are able to put your whole hand into this space.
If a cinch is too short for the horse's barrel but the saddle fits and you like it, the saddle can still be used. Borrow a cinch that works to properly fit the saddle and then purchase a longer, replacement cinch.
Other factors to consider when choosing a saddle include the purpose of the saddle and the type of horse. Saddles used for roping must be durable and able to handle a large amount of torque, while saddles for trail riding must be comfortable. Some saddles are also suited specifically for certain types of horses. Mule saddles have a straighter bar, making them suitable for a mule's back.