Fitting the Rider
Generally speaking the bigger the rider, the bigger the saddle. But the real question is bigger how? French saddles are stamped according to the seat size, which is measured in inches diagonally from the 'button' (or nailhead) on the skirt of the saddle to the center of the cantle, and flap size and shape. With respect to seat size, the rule of thumb is to be able to fit three to four fingers sideways behind the rider to the cantle. This, however, can vary depending on the rider’s personal preference.
Seat size and seat surface area are different though. All 17" saddles are not the same. For example, 17" Devoucoux Socoa or a Butet has a narrower seat where the seat bones contact the saddle than the 17" Devoucoux Biarritz, which may be up to an inch wider. So the broader hipped rider who is the same weight and size as a narrower hipped rider may prefer two different brands, or models within the brands, despite having the same seat and, potentially, flap size.
Flap size is composed of 2 components, length and forwardness (or shape). Length is measured from the stirrup bar along the fall line of the stirrup leather to the bottom of the flap and is indicated by a number, such as 1, 2 or 3. The smaller the number the shorter the flap, but a '1' is not necessarily the same across brands. Forwardness is indicated by a letter such as 'A' or 'AA' and is achieved by making the flap wider, with the extra width coming in the front of where the stirrup leather would lay. Different brands achieve forwardness with somewhat different shapes of flaps and have differing degrees of forwardness available.
Longer legged riders need a longer flap while shorter legged riders need a shorter flap. Riders who are particularly long from the hip to knee, or ankle to knee will often need a forward flap, that may or may not also be a longer flap. The general rule of thumb (and a good place to start), is the bottom of the flap should go about 1/3rd of the way down the calf and the flap should end at the widest part of your calf. The bone on the inside of the rider’s knee should rest on the lower 1/2 - 1/3 of the knee pad and place the rider’s knee cap close to the front edge of the flap, but never over.
Here is an interesting comparison of Amber, who is 5' 8" and wears a size 26" breech in 2 different 17" saddles. In the first photo we see Amber in a Devoucoux Socoa 17" with a 2 flap. While the seat looks great the flap is small overall. It looks a hair short and she needs bit more knee roll in front of her knee. In the photo on the right, we see her in a CWD SE01 17" 2C flap which is the forward flap. The seat looks too big and the flap is the wrong shape for her leg and maybe a bit short. While both saddles are a 17" seat the surface area of a Socoa is less, so the seat looks right, however the best flap for her would probably be a 3A - slightly longer with a touch more forwardness. In the case of the CWD, she could either select a different seat model, or go down to a 16.5", and go up a size in the flap to a 3 and select the standard L shape, which would fit her leg better.