How can you tell what size saddle I need?

Answer 1

The stirrup iron should half a quarter to half inch of space on either side of your foot.
The stirrup leather should be adjusted depending on the sport you're participating in. Dressage requires a long straight leg, jumping uses a short stirrup leather, and general or trail riding uses a length in between dressage and jumping.
To adjust for these make sure the leather is attached to the saddle and then pull it out and away, it should be roughly as long as your arm and may need to be adjusted up or down a couple of holes for various disciplines.

Answer 2

not being rude, but it depends how big you bum is and how long the horse/pony's back is. You need to get a saddler out to fit a saddle. Also, you can ask for a child's saddle, a small adult saddle and a large adult saddle.

Answer 3

To tell how long your stirrups should be you have to have the stirrup attached to the saddle. It should be as long as from your hand to your armpit.

Hope this helps :)