It's quite simply a big hunk of thick full grain leather. 1-1/2" wide with antique brass hardware. The length is measured from the heel bar of the buckle (where the leather connects) to the middle hole at the other end; see diagram.
Typically, add 2" to your pant waste size. So if you wear a 34" wasted pant, you'll need a 36" belt.
If you want a custom size, state your request in the notes when ordering.