I have the tri-zones. Haven't tried them XC yet, but they were a bit snug on my big-boned WB mare (I bought them on eBay, so no return), but doable. My thing is the bulk upgrading from the neoprene Woof boots I'd been using - they're HUGE...
"Adulthood? You're playing with ponies. That is, like, every 9 year old girl's dream. Adulthood?? You're rocking the HELL out of grade 6, girl."
I broke down and bought the Dalmars at Rolex after talking to the Bit of Britain folks. They were very helpful.
I LOVE these boots! They have what I call a strike plate at the front and back. They breathe and are quite flexible, with great coverage. They do not hold water. I got the largest as my pony is quite stout in the leg area. They fit him, although a tiny bit small in back, and my trainer can also use them on her much smaller boned TB. They are very pricey, but frankly I don't plan on having to buy another set of boots ever. My trainer has been very impressed with the quality of these and now wants a set for herself. I think we will end up sharing for a while due to the $$.