You have to be careful with larger sized western saddles. When you start getting past 16 inches, the load gets too far aft on the horse's back and makes the tail of the bars start to poke the loins. Can make a horse pretty uncomfortable. If you need a 19" western seat, you also need a large horse and possibly a custom saddle. A 19" Ralide tree just isn't going to be good for the rider or the horse.
I stand 5'9" and weigh in at 185 pounds, average build. Now that's not huge, but I could stand to lose 15 pounds. I sit comfortably in a 15" western seat. I can sit comfortably in a 16" cutting saddle or roping saddle, but I start feeling a little bit like I'm sitting on a table after that. I prefer a 15" saddle with a tall cantle for most riding. It's a little different for men and women, since they tend to carry body weight in different locations, but the fact remains that when you start getting up into the higher numbers of seat length, you start having saddle fit problems on the horse's back.
There's a great site by an excellent custom tree builder at http://rodnikkel.com
. He has put out some great articles, in the form of blog posts, on his site touching all aspects of saddle trees. It's a good read.
But to "plus-one" on the original question, Aussie and English saddles measure the same way, which I believe is between the highest points on the pommel and the cantle, but Western saddles measure seat length from the base of the horn, where it transitions to the pommel, to the seam on the top of the cantle. In general, a 15" western equates to a 17" Aussie or English. Having said that, saddles of the same measurement from different makers may fit differently on a given individual or horse.