Shed skins give you no indication of length. They are useful however for purposes of comparison. I keep skins from my animals when they come out perfect, and I compare them to later skins that also come out perfect, and that way I can sort of gauge their growth. The best way to measure a snake, to me, is to just watch it crawling around the cage, and as they line themselves up along the edges and around corners, notice their body length and where they stop/start. You can usually use a measuring tape or whatever to measure the cage. Otherwise, I agree. Just use bodyweight, because length is really impossible to determine accurately on such wriggly animals.
--Naamah