I'm not sure if you mean english or western (or if it's all that different) but for an english saddle you measure from the little metal button almost at the front of the saddle to the center of the cantle. That measurement is your size. From there you can also get different lengths of flaps or forwardness of flaps (for jumping saddles). The more custom saddles will have even more options. The width is measured from the front of the pommel. I usually measure from metal button to metal button. Different brands will measure "regular" differently. Figuring out what your size will depend on your thigh length, size of seat, etc. And since different brands fit people and horses differently it's important to go and try as many different kinds as you can so you can figure out what works best for you.