I usually don't bother with measuring length, I'm usually more concerned with weight in which case I use a digital scale. But when I do need to get a length, I usually just take a piece of string and hold one end of it on the snakes head and then while holding the rest of the string run my hand down to the tail of the snake. I hold the string where it meets the tail tip and then just measure the string with a tape measure. Maybe not THAT accurate, but close enough.