English seat sizes are measured differently than Western. The measurement is taken from the nailhead at the side of the pommel to the center of the cantle. The seat size you need is related to both your femur-length and butt-size.
Seat sizing: http://www.evideolibrary.com/calcula...lator_eng.html English saddle fit
You'll need to make sure the saddle is the right width and profile shape for your horse as well.
Check out the 9 Schleese videos; they'll give a very good intro to fitting the saddle to your horse: http://www.horseforum.com/horse-tack...lly-fit-58116/
Having a saddle-fitter is best. If that's not an option:
For the saddle, take a wither-tracing. Get a flexi-curve or a straightened-out coat-hanger (flexi-curve is most accurate and easiest). Lay it over your horse's back about 2" behind the rear edge of his scapula and press down to get a "casting" of his back. This is the place the tree-points on an English saddle should sit (approximately.). Now trace this onto a piece of cardboard and cut it out. Now you can take your "horse" saddle-shopping with you. When you look at an English saddle from the front, lift the flaps, and you'll see the tree points there, right in front of the billets, tucked into leather pockets. If the angle of the tree points matches the angle of your "horse", you have a good start. If you're buying online, look for a seller who will let you return the saddle if it doesn't fit. Look at your wither-tracing. Around 90° is about a medium. Wider is a wider tree, narrower is a narrower tree. All manufacturers have a completely different idea of what the various tree widths actually are, so you really need to try the saddle on your horse.
That's just the width. Remember the shape of the saddle from front-back has to match your horse's back as well.
Make sure that the saddle you get has good panels on the underside. They need to be soft enough to conform to the horse's back and provide cushioning, yet firm enough to provide support. No lumps are allowed. A question about measuring