A really good way to fit a saddle well to a horse its to take a piece of heavy wire(about 2' long) and bend it over the withers to fit the shape of your horse. Then take another wire and bend it over the middle of the back. Trace both to paper and measure the width and height and that should give you a better idea of what you are looking for.
You can also trace them to a piece of poster board or cardboard and take them with you if you go to check out a saddle in person and that should show you how well the saddle would fit your horse.
With your horse I would definitely look into gaited saddles.