Seat size is measured differently and is scaled differently for English and Western saddles.  All types of English saddle seats are measured from the metal rivet (or screw) on the side of the pommel (front top)  to the center of the top of the cantle (the raised back portion of the saddle).  A hunter/jumper seat sized for a typical child's saddle will be 13 -14", a youth or small adult will be 15 - 16", a regular female or slender male will be about 17", and a male or large female about 18".  If a person is very tall they may want to go with an extra inch of size to let the knees lay on the flaps properly.  A cut back (saddleseat) saddle' s sizes will generally run about 4" larger, ie:  a youth's saddle would run about 19 to 20" for proper fit.  Dressage saddles tend to  run an inch or two larger than a hunt saddle,. but you need to be careful here - if the seat is cut very deeply it will feel very differrent from one that is cut a little shallower.  The same holds true to a lesser degree for Western saddles.  They are measured from the inside top of the cantle (the back) to the inside top of the pommel right below the horn.  A regular female is most often quite comfortable in a 15" seat, and an average male in a 16" seat. Again this is only a general rule of thumb...a wider seat may "sit" bigger than a narrower one and depending on how deep the seat is dwill affect the rider's feel.  Before you shop - sit in as many saddles as possible to find the style and seat size that best suits you.