This is what Ebay
says about it:
I have found a lot of sellers that have been measuring their English saddle like a western saddle. This is incorrect. If you measure your English saddle from the center of the gullet straight back to the cantle, the size comes out way too small. To measure an English saddle, go from the brad or nail head at the side of the pommel [front of the saddle] to the center of the cantle [back of the seat]. This measurement will be at an angle. This will give you a correct seat size.
When purchasing an English saddle, most people think the seat size is all about their size, but the most important factor is not your fanny size, though that plays some part in fit, it is mostly the length of your upper thigh. If you purchase a saddle that is too big for you, you will have a very hard time ever maintaining a correct seat position, you will constantly have to fight to avoid riding chair seat, and your posting will not be the small, smooth movement it should be, but rather a huge effort at popping up and down in the saddle. When you sit correctly in any saddle, [with the exception of a saddle seat saddle], you must have a line from your shoulder to your hip to your heel.