Fitting the english saddle is NOT the same as fitting western, and gullets are different. In fact it's much harder to fit english saddle (in my experience). So my suggestions are either call saddle fitter out (as already suggested) or do the tracings and talk to the fitter or tack store over the Internet. If your saddle doesn't fit you may damage the horse's back in long run.
As for size, I rode in 15" western, but my dressage and jumping are both 18" because I'm tall with long legs. This is the calculator on what seat size you need: English Saddle Seat Size Calculator