Use a string and lay it on the snake, provided it's a mellow animal or at a resting state. Another if the animal is restless is to get a long clear tube slightly larger than the thickest part of the snake and coax him through. Another, but risky method employed by most to include me is to attempt to grab the snake and position him/her next to a tape measure layed out on the floor. Gently grab it near the neck wrapping your WHOLE HAND around to reduce the possibility of injury and that will put your pinkie finger furter down the neck as well, where the chance of injury is unlikely. Put your other hand near the base of the tail, but not the tail itself. Now, while the entire length of the snake is supported by the floor, gently straighten it out, but ONLY use little effort (otherwise you could fracture the vertebrae). If it resists a lot than let it loose and try again. After several attempts, most snakes will relax a bit more, allowing for a pretty accurate measurement, from head to clocoa. The tail can be measured later by quickly taking it and positioning it next to the tape after you grab the snake near the last half and base of the tail. Do this quickly, it can usually be done before the snake pulls it's tail away in a hurry. Then just add the two measurements together. I have done this myself for years on thousands of snakes to include venemous ones and NEVER injured one yet! Just make sure you don't get frustrated, have patience and it'll work everytime.
Good luck with it. Hope this helps.
Zee