How do you measure bellows?

Hi,

I feel a bit silly asking this question, but how are bellows measured?

To my eye, there are four measurements that seem obvious:

1) The internal size (the empty space)
2) The external size of the bellows themselves (frame not included)
3) The size of the frame into which the bellows fit
4) The extension length of the bellows when fully extended.

I have the biggest problem with #4. How do you take something as stretchy as bellows and make an accurate measurement of length?

I'm a newbie. Is there anything I'm completely missing the boat on? Advice and "Doh!" comments both welcome :-)

If you're having bellows made, supply them with the internal and external frame sizes.
for #4 you either look up the specs for the camera you have, fully extend your camera and take a measurement from the front to rear standard