Snake sheds can stretch by more than 25%. So they are not an accurate way to measure the length of a snake.

There is the string method. Where you take a string and run it down the center of the back from the nose to tail. Then measure the string.

You can allow them to crawl along a wall and mark the head and tail. Then measure the marks.

There is also a program which allows you to take an aerial picture with and object of known size and it will figure up the length for you.
http://serpwidgets.com/main/measure