In a tack store you might see a 15" saddle, 16", 17", 14"...
The measurement listed is the length of the seat from pommel or horn to cantle. Western saddles are shorter because of the slightly different way the rider sits. (An English saddle is about two inches larger for the same size rider).
Seat length is also affected by discipline, especially with western saddles - barrel racers and ropers choose shorter seats for extra security.
English saddle seat is generally assessed by the upper leg length of the rider, whilst western saddles start with the rider's height and weight, but saddle fit has a lot of other factors too, for both the horse and the rider.