Saddle Fit and Design for the Short Rider

size depends on the circumference of the riders hips and upper leg. It can also depend partially on the length of the horses back and belly. Discussing seat size is a completely different topic which also concerns the pommel cutback quartercutback or not as well as tree width.
If you are a shortlegged rider choose a saddle with a narrow twist a medium to wide waist a wide seat and a half panel in addition to a shorter flap. Consider using a professional saddle fitter to help you find the right saddle that not only fits you but also fits your horse. The pommel has nothing to do with the rider its designed to accommodate the horses withers. There are several areas of tree width you can consider but again most of these have to do with fitting the horse not the rider. 
Just remember that it is extremely important that you the rider are comfortable in your saddle otherwise no matter how well the saddle fits the horse he will never perform to his optimum potential because your discomfort will translate down through the saddle to your horse.