This is because all land vertebrates share a common ancestor. All land vertebrates emanated from evolution form the oceans and seas from a group categorized as Sacropterygii. Therefore during the early phylotypic stages of embryonic development, the embryos of all land vertebrates look alike morphologically. It is believed it is during this stage that the basic plan of the body takes shape before it begins to diverge.
Inches or centimeters, because it is not that big so you would not use anything else because its not logical.