Typically you measure your thigh to figure out what seat length you need. Sit in a chair. Measure from your rear on the side to your knee. Try a bunch of saddles, even if they are too expensive. Then when you find what you want, you can go online and shop for used ones that are less expensive.
Goodluck to you. Finding the right saddle is very important for balance, function and comfort for both you and your horse. BTW, he looks so much happier in the snaffle!