I agree with PocketPony in that I feel that Western saddles are much harder to fit because you cannot see as well what is going on with the tree, and you cannot make modifications to a Western saddle like you can to an English.
If you know how to fit an English saddle, the basic fundamentals are the same. So it’s more of educating yourself on the different parts of a Western saddle and how those should complement the horse. Here’s a few websites with great info:
http://saddlemakers.org/id193.htm
http://www.rodnikkel.com/content/tree-and-saddle-fitting/factors-that-affect-tree-fit/
Also, there is NO STANDARD in the Western industry. You can have two saddlemakers that both have a full quarter horse bar tree with a 6.5" gullet … and they can fit 100% different.
It’s simply trial and error. You can kind of “know” if your horse is going to need a semi or full, to give you a starting point, but you just have to try on saddles.
My rule of thumb
- Fit the horse
- Fit the rider
Of course, you can have the best fit in the world for the horse, and it doesn’t do you any good if it doesn’t fit the rider. It’s got to fit both.
For the rider, a general rule of thumb is that you want 2 or 3 fingers from the thigh to the swell, as this picture on Google depicts.
Roughly, a Western seat size needed will be 2" smaller than the equivalent English saddle. Again, general guideline. So if you ride in a 17" English, you’ll want about a 15" Western.
Also for the rider, you want the saddle positioned for the what the rider is going to do. There are differences between a trail riding saddle, or a cutting saddle, or barrel racing, etc. So the type of saddle will also depend on what the rider wants to do, and what body/leg position will work best. It’s really not always a perfect heel-hip-ear alignment, depending on what you are doing.