Let me confuse you a little more, Jim.
I ride English, and the seat sizes for English saddles are measured completely different than those for Western. We measure them from the nailhead on the pommel, diagonally across to the back of the cantle.
A 15" or 16" seat in an English saddle would be considered a youth saddle. Adult saddle sizes in English are considered 17" seats and up. I ride in a 17.5" seat. 17" to 18" are the most common sizes for adults riding English.
The girth lengths are also different sizes, depending on the type of saddle. A dressage saddle will take a shorter girth than a jumping saddle, due to the length of the billet straps.
It's nice that you're trying to learn for your granddaughters.