I have a set of these, they're not great. They are very lightweight and easy to put on but I have found that mine rub. I have a 16.1 thoroughbred mare, have med on the front and large on the back but they don't fit very well (hence the rubbing). The straps are really long and the vents don't match up on the front ones as I have to overlap the boot so that it (kind of) fits. The actual length of the boots is fine so they may fit better if your horse has more bone, I have gone back to my double velcro WW boots.
Shame because I have some of their saddle cloths and rugs and they are great. Hope this helps!
I have the normal XC boots and use medium on my boy (15.3 and quite dainty legs). I find them great and such fantastic value. I just bought the fetlock boots and a tail guard from them too.