Information from Horse.com https://www.horse.com/content/english-tack/guide-to-english-saddles/
Measuring your Saddle Seat Size
The easiest way to measure for the appropriate seat size is to measure your upper thigh.
- Sit in a chair with your feet flat on the ground so that your knees are bent at a 45-degree angle. Be sure your buttocks are touching the back of the chair.
- Take a measuring tape and run it along your thigh bone, measuring from the end of your knee to the backside of your buttocks.