Typically Western saddles are 2 sizes smaller then english.
So if you’re a 17 english, you should be a 15 western.
The best thing to do though is go to a tack shop and sit in a 15 and a 16. you might find that you’re more comfortable in a 16.
you should have a hands worth of space between your thigh and the cantel.(at least that’s what I hear) I go by what fits my butt and my horse.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."