The shed skin will be MUCH larger than your snake, since it stretches when they take it off.

The easiest way to measure is to use a string, like Jenna mentioned above. If you can have a friend help, that's even better.
Start at the nose and then slowly work it down their body, until you get to the tail.

You can also check out the Snake Measurer program. All it takes is a picture of your snake next to something like a ruler.
You then calibrate the program to the length of an inch (or whatever measure you find easiest)
and draw a line on the picture of the snake. It's pretty cool.

http://www.serpwidgets.com/Apps/measure.html