David,
When we were ordering our Kirkey seats, I just used the wood stairs in my house and a tape measure. When sitting on the steps wearing jeans, I had my Dad measure my hip width from the wall. It was just under 18" so I ordered an 18" seat. With Kirkey vinyl cover on the seat, I am snug in the seat and secure. Additionally, here a link about measuring yourself for Kirkey seats.
https://pitstopusa.com/p-4240-kirkey...uidelines.html
Good Luck,
Andy