Mens size 21 boots and shoes for big feet. When it comes to large size mens footwear, only XLfeet is constantly seeking out and making available mens shoes and boots in size 21. Men rely on XLfeet for size 21 mens socks. We have women who buy size 21 boots for their kids, and wives who buy size 21 shoes for their big feet husbands as well. We have these size 21 boots from Bogs, and socks from Extra Wide Sock Company.