GRRRRRREAT question!
Mine lives in an 18" pipe and hangs out the front and back an inch or so on each side, so the best I can figure is 20"ish! No way I'm going to try to lay that beast on a tape!
At least I know the pipe is 18"...... Pipes don't bite!
By the way, if someone thinks they have a big fish, just take a picture of it. Blow it up on your computer until it prints out at "X" inches in length, and then go tape it on your tank. Do that until you get a 2 foot dovii (yes, it will take more than one sheet of paper) and you'll likely see that a 24" dovii is muuuuuuuuuuch bigger than just about any you've seen!