When selecting a western saddle, your saddle seat is important.  Your western saddle seat size is about 1 inch less than your english saddle size.  So, get out your measuring tape and start at the middle of your knee on the outside, to the middle of your hip.  Then subtract ONE inch.  Now this typically works for the adult middle aged female, 2 inches works for the teenage-twenty somethings...  This is where it gets fun.  As a 14 year old girl weighing about 118 pounds, I rode a 14" Big Horn saddle.  In the 1970's where my weight went from 118 to 105 pounds a 14 1/2" seat was FINE.  Then the 1980's and 1990's happened...  I went from a 14 1/2" seat to a 15 1/2 " seat without realizing it.  Good thing the foam was breaking down in the seat of my 1969 Circle Y.  Upon reviewing old pictures, I can now noticably see that my working saddle had become a "chair".  Maybe it was a good thing that in 1988, somebody broke into my tackroom and stole my 1969 Circle Y, forcing me to have Broken Horn make me a working saddle.

Typically, when measuring yourself for an english saddle, you measure from the middle of your knee to the middle of your hip that should give you the proper seat size.  Hence if you're a 16.5" you'd use that for a huntseat saddle, add and inch for a dressage saddle and take away an inch for a western saddle.