English Saddle Seat Size Calculator
is a seat size calculator. Thigh refers to thigh length, and you sit up against a wall and measure thigh from the wall to you knee. Or it's something like that. It takes into consideration how much booty room if you measure it that way: butt to knee.
Or, take your western size, plus 2" and that will roughly be your English seat size. An open or flat seat with low cantle will be more roomy than a deeper one. U-shaped seats have more "sit" room than deep seats that are V-shaped and have a steeply sloped cantle. A hunt seat saddle with a deeply cutback pommel will ride or feel shorter than one w/ less cutback pommel. The saddles like Duett Allegro and Ashley & Clarke Ayr ride smaller -- I like 1" larger than my normal 16.5" cc size in the Ayr and 1/2" larger in the Allegro. I posted pics of those and other English saddles at Horse Grooming Forum. I'm Tack Collector over there, too.
Dressage saddles that are very deep - you might need a large size like 17.5" to 18". It all depends on your preference, how you're built, how much room you like, and also what saddle fits the horse and sometimes the horse dictates huge compromises in terms of the model of saddle that it's feasible to use.