The size of the saddle seat needed depends on the size of the rider.  Here are a couple links to our website that will help you select the proper saddle seat size.

Western Saddle Seat Sizing Chart:  http://www.horsesaddleshop.com/saddle-seat-size-chart.html

How To Measure a Western Saddle: http://www.horsesaddleshop.com/howtomeaswes.html