What kind of riding you choose to participate in will determine what style of English saddle you buy.  English saddle designs are based on physics.  Where the stirrup bar is in relation to the deepest part of the saddle will influence where your leg hangs below it.  The farther forward the stirrup bar the more in front of you your leg will be. All-purpose saddles are usually designed this way.  It is a far more protective, defensive riding position.  It takes longer to fall off forward if you leg is parked out in front of you.  No matter how hard you try you will not be able to put your leg under you for a proper equitation position in an all-purpose saddle. Riders on all-purpose saddles generally ride with shorter stirrups. The flaps on all-purpose saddles are set farther forward in order for your knees to not stick out off of the flaps.  Riders compensate for this by leaning a bit more forward to balance. This type of saddle usually had a round cantle, a deeper seat and knee rolls.  All-purpose-saddles are used mainly for trail riding, recreational riding, fox hunting and cross-country horseback riding. These kinds of saddles are comfortable for long rides.

English Saddles are measures from the button on the front of the saddle to the center of the cantle. The measurements are in inches. Adult saddles are from 16” – 21” every half inch.  A correctly fitting saddle will leave you room behind your bottom measuring about the width of your hand.  There should be room in front of your crotch to allow you to comfortably post with out hitting the pommel of the saddle. Also, be sure that your knees do not pass the front of the flaps when you are wearing your stirrups.

Be sure to check the underside of your saddle to see if the panels are wide enough to support the weight of the rider. Narrow thin panels can cause back trouble in horses.
Choose a based on you riding style and goals.  Have a professional saddle fitter help you if you have problems fitting you horses back.