My suggestion is to find a Professional Saddle Maker/Fitter to come out and help you with this saddle search process. There is a lot to ensuring proper fit for both you and your horse.
A good saddler, will come out to your place with a truck/van full of saddles, that he/she will assess on your horses back to ensure proper fit, and then will see you ride in the saddle to see how you are positioned, and how your horse reacts.
They usually cost around $100+. Find a saddler who doesn't work for a particular company aka county, stubben, etc, etc.
Also, you can go to a tack store, and have the saddler put you in a bunch of different english saddles, to see how they fit you and what position they put you in. Each saddle will put each person that sits in it, in a different position due to our conformation.
It isn't just about seat size, it's about the twist of the tree, the forwardness of the flaps, the position the saddle puts you in - I can sit in one saddle, and it can put me in a chair seat, pushing my seat to the rear of the saddle. I can then sit in a different saddle, and it'll put my feet directly under me, my weight will be distributed near the pommel and I can feel instantly stabalized and balanced.
You wont know, until you sit in them.
Then, the saddler will put the one's that work for you, and "may" work for your horse to the side, and then will bring them out and try them on your horse first to ensure proper fit, and then will continue onward with the process.
There is also a thread I started in the Tack And Equipment section, titled "Does your saddle really fit?" and you can watch the step by step process of saddle fitting for your horse. Schleese made the videos, which are very educational and helpful. There is A LOT to saddle fitting - far beyond what size of tree should you get.
I wish you the best!