I was referring specifically to the woof double locking brushing boots for determining the size by the # of straps because you can with them. And since the thread is called Help! Woof Double Lock Brushing Boots...
At the bottom of the Dover page you can see they specify the # of straps the sizes have:
Woof Double Lock Brushing Boots - Dover Saddlery.
But, the other single-lock woofs you guys mentioned only come in 2 straps, no matter what size.