When selecting a Western saddle, it must fit you correctly so you can maintain a proper seat and it must fit your horse so that he can perform comfortably without developing soreness or injury.
Fitting the Rider
The seat size on a Western saddle is determined by measuring the saddle from the back of the horn to the top of the cantle and ranges from 12 to 18 inches. When you sit in a Western saddle, you should be able to fit two to three fingers between the front of your thigh and the fork of the saddle. You should be able to fit approximately four fingers between the back of your seat and the top of the cantle.
Saddles seats also have different widths. Slender riders with a narrow pelvis will be more comfortable in a narrow seat. Heavier riders may feel more comfortable with a wider seat.
Fitting the Horse
Determine what tree size your horse needs. Many horses with defined withers do well with a regular tree. If your horse's withers are wide or rounded and he has a flat back, he needs a wide tree. Draft horses need an extra wide or draft tree. Use an extra pad to fill in the space if a horse has narrow withers.
Determine if you have the correct tree size by placing the saddle on the horse's back. If you can place two to three fingers between the gullet and his withers, this is a good fit. If the space is larger, the tree is too narrow and if you can only fit one finger or less in the gap, the tree is too wide.
The bars of the saddle are the pieces of the tree that sit against the horse's back on either side of his spine. The bars should be at the correct angle so the bar provides even pressure on the your horse's back without gaps. Proper fit is critical as the bars help distribute your weight evenly when you are riding.
Considerations for Gaited Horses
Gaited horses, such as Tennessee walkers and fox trotters, may need a tree designed for a gaited horse. These trees have a higher gullet that is common in these breeds. Additionally, they have a flare in the front bars to give the horse's shoulder greater range of movement and more rock, or curvature, in the bars to conform to the horse's back.