I'm 5'7" size 29/30 breeches. I ride in an 18" CWD medium deep seat saddle. Could have been a 17.5", but went up 1/2" for a bit more forward flap. I am ridiculously long from hip to knee. I also ride in a 17" Jaguar which has a deeper seat.
Depending on the depth of the seat, you can generally go up or a down an inch.
I am 5' 5" and currently ride in a 17.5" deep seated saddle, I have ridden in 16.5" flatter old style collegiate very comfortably though. The 17.5" fit me (I was told by my trainer) but I prefer the feel of the 16.5" or 17".
I'm 5'8, no butt, but long legs. I ride in a 17", medium deep seat, with a narrow twist. I fit better in a 16.5", but I have the 17" because the flap fit my leg better. Saving up to get a custom saddle right now.
5'3 with a SHORT leg (I need children's stirrup leathers) and a rather large badonkadonk but wear 26-28 size breeches and I stick to 17". OLD stubben and Bates 17" fit wonderfully. Just got a 17" Tad coffin and its a taddd scrunched but it is fine
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I'm 5' 4 1/2" and have short legs. Pants are 29 inseam, but my lower legs are so short that my femur ends up being pretty normal. I like a 17" saddle with a flat seat and not-too-forward flaps. Plenty of room behind, but I carry my weight in my hips as opposed to my butt.
I am 5'4" and 145 pounds with a 29 inch inseam. I am not a skinny girl but am relatively proportionate all over. I tested out a bunch of saddles awhile back and my best fit for a jumping saddle was 17 and for dressage was 17.5 across several different brand. Like someone else said, I found the twist to be in the biggest issue. I need a more narrow twist to be truly comfortable.
I ride in a 16.5". Most people my size probably wouldn't fit in this size saddle well, but apparently I 'don't take up much room when I ride' haha. I love my saddle. It's a flatter CWD. (I'm 5'4, was in a size 32-34 breech when I bought the saddle though am currently a 28-30 sized breech, semi long leg for my height.)
If I were sizing you I would ask what you weigh and your femur length. Both of those saddles have an average forward flap and someone of your height can be in a 17 or 17.5 depending on your answer to the above questions. The main thing is that you feel centered in the sweet spot in the saddle. If you are riding up on the cantle it will influence your balance. Sometimes, a bit bigger seat will allow your leg to meet the flap edge properly and sit centered. If you have a short thigh then a 17" might be better. That is the beauty of a trial policy as you can try both if available and feel which one works for you. Trumbull Mtn carries both of those brands and does offer a good trial policy.
5'9", long femured (but most of my height is actually in my back), 17.5" Bates with a forward flap is my current comfy saddle. Most of my body weight is in my midsection (35" waist), but somehow I've got plenty of room, without swimming.
18'' with forward flap Dev Oldara. It has NOTHING to do with butt size or weight, unless we're talking huge extremes. I have a friend who weighs 300, and her AP is an 18''. It might be a little on the small size, but doesn't interfere, because she rides with a straighter leg whereas I prefer to jack my stirrups up up UP.
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