If you can find them, Dafna’s go pretty big. I take a 17" in a normal dress boot, so add wool socks and two or three layers… the Dafna’s still mostly fit. They have four velcro closures–like splint boots! There is one I can never get, but I just hook it back to itself and don’t worry about it.
I’ve no idea where to find them though, I got mine at an Expo, then on Ebay.
Brit Riders by Muck Boot are good. Not perfect, but good. They will stretch to accomodate up to around 20" I think. With the right socks, they go down to single digits quite comfortably. They help riding position very nicely if you’re a dressage-boot type. I only had mine 2 years though before the outer layer started coming apart from the inner layer a little–which leaves room for stuff to get in there (mud etc.). They also are ‘peeling’ a little on the shafts. For $100 for RUBBER boots I expected a little more longevity. Still, they are warm and they fit. IF they get wet though inside, you must have some sort of boot dryer or be able to put them over a heating vent if you need them to dry in any reasonable time frame.
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