Take a string and run it along the snake's body and hold it where you reach the end of the snake.
Now just measure the string.
There is a software program out there that allows you to measure your snake from a photo. You have to get a photo from directly above and with something for size reference. Then open the picture and click the reference and then click along the snake's body. I'm sure it was a free download from somewhere.
If there is anything the nonconformist hates worse than a conformist, it's another nonconformist who doesn't conform to the prevailing standard of nonconformity.