[QUOTE=Madeline;8422498]
What really would help would be if saddlers would add adjustable stirrup bars to their options. I had a horrible time finding a saddle that worked for me until I started putting a 1/2 or 3/4" bit of radiator hose on the stirrup bar. That changed the geometry enough to get rid of the chair seat tendency. And it cost less than a dollar.[/QUOTE]
I do this on a couple of my saddles with rein stops, and it can definitely make a difference. I am a card-carrying member of the long-femur club and I would LOVE it if long/adjustable stirrup bars would catch on. A long and/or forward flap gives your thigh a place to go without having to upsize the whole saddle to achieve the same effect. I have bigger problems with chair seat when I ride excessively large saddles than I do with a custom flap.
But keep in mind that human geometry is highly variable and a saddle that causes horrible chair seat in one rider may not in another. If your femur is a lot longer than your lower leg, for instance, you may be able to find a saddle with a nice forward/long flap to accommodate your thigh but your heel is just never going to line up with your hip and ear a la George Morris. Which is fine - all you need to do is find a saddle that feels good to you and provides a balanced base of support from which you can ride effectively. But what works for one person may not work for another? Sadly this means that there are no short-cuts to finding that perfect saddle for you - you will have to try them for yourself and compare.
I don’t think 17.5 sounds huge for someone of your size with a long femur, but then again I prefer a roomy saddle. But some 17.5" saddles have open seats within which you can find multiple different positions while others of the same size have a single sweet spot. So the fact that you feel like you’re swimming in yours and struggling to find a balanced spot could have more to do with its design than its size. A good saddle fitter or your trainer should be able to help you with that.
For me personally, I must have a long/forward flap in jump saddles, and I also prefer a forward flap in dressage saddles. Some jump saddles like this DO put me in a chair seat, the English types in particular. The forward-balance type saddles work best for me, and I love my Prestige Red Fox.