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 SE03 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.