Meaurement of saddle size is fairly standard across the industry. If you are looking to purchase a particular brand of saddle that you think may differ, check with the manufacturer or tack store, and they can give you the measuring points.
In general, what you are measuring is the seat size. For a Western saddle, that should be measured from the base of the saddle horn straight back to the center of the cantle.
For an English saddle, the measurement is from the saddle nail, which is on the front of the saddle near the pommel, to the center of the cantle.
