XXXL overreach boots

Big Horse Shop.
My Suffolk wore the 4XL when he needed them. In terms of size, the diameter of the bottom of the boot is the same width as H&H magazine, you could wear them as a hat.
 
3xl don't exist (I have researched extensively).
In 4 xl you have 2 options: Big Horse Shop or Davis Draft bell boots. The Davis ones are much more robust but i expect more expensive also.
As O&L says I have worn them as a hat previously! My clydesdale was in m2m shoes and they just about fit him. If you just have hunter feet you may find them too big.
 
Top