I am looking to get a new cutback english saddle, how do I measure to tell what size I may need?

I know that a 20" is too small, I am about 5'9".what's the rule of thumb? What's the difference for western saddles? How do I measure for that?
Answers:
Most tack stores will let you sit in the saddle to find out. They're also pretty good at telling what's too small and too large. I knew a few places that let you take a saddle for a trial period as well, which I love; I think you really need to use it before you can make a decision. I think Dover saddlery actually does this through their magazine if you can't make it to a tack shop.

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it all depends on the size of your butt.
Most saddle shops will let you sit in one to check for comfort

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I am 5'8 ish and I would say try a 21 -22 inch for your cutback
You measure a western saddle from the center of the horn to the ctr of the cantle - very similar to a reg.English saddle - dressage ,AP,huntseat sadde - I would try an 18inch.

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You need to go to a tack store for that answer.