How do i measure my saddle seat size?

the best way to measure is by actually sitting in one. About a palm from your bum to the back of the saddle should be plenty. You don't want one too big or too small, also, if your sitting in it with correct length stirrups and your knees go over the flaps, it's too small. Try a few saddles at your stable, maybe someone will let you sit in theirs and give you their own opinion, most people are happy to help!