When measuring for a sidesaddle, there are five measurements needed to ensure the best fit possible. (The best way to obtain the first four of these measurements, is to drop your leg over the end of a picnic bench.)
1. Seat length. Mark behind the seat, and then measure the length of the leg, from behind the knee to the back of the seat. This corresponds with the measurement of the saddle from in front of the horn to the cantle.
2. Seat width. This should also be measured in the "sitting" position. Mark the left and the right sides of the hips and measure across. This corresponds with the width of the seat of a sidesaddle.
3. Height of the knee. Measure from the top of the bench to the top of the knee. This corresponds to the height of the horn on the saddle.
4. Circumference of the knee. Measure around the knee. This dictates how far out from center the top horn is to be set.
5. Type and size of the horses back and withers. The best way to accomplish this is to make a cast of your horses back to send to the saddler or seller of the saddle your are considering using. Instructions on how to make a back cast can be found here.