tubing length and proper sizing

. What is the proper length of tubing?
There really is no rule here. In my opinion, shorter is better, because it allows more sound pressure to reach your mouth from the driver. You may find that very long tube lengths reduces the effectiveness of your talkbox, making you have to turn the volume up, and adjust the EQ of your amp to have a more robust sound. The bottom line is, use only what is necessary to get the sound from the driver to your mouth, with the talkbox sitting on the floor. 

10. What is the proper size of tubing?
Once again, there really is no rule, but the two sizes that are most commonly used (and the only two sizes that I recommend ever using) are:
5/8" O.D. (outside diameter) and ¼" O.D.
The most common size is the ¼" OD, and that is what I recommend to beginners - it is easier to get clear diction with that size of tube in your mouth. In the US, you should be able to find WATTS plumbing parts at any home supply store. Get the yellow color-coded tubing. Buy plenty - you will always want to have extra, because it’s a good idea to change out your tubing once in awhile. 
The larger size, 5/8" OD, is nice for a fatter tone, because more air goes through the tube, and just the physics of a larger column of air yields a slightly different harmonic balance. I find this size harder to talk with, but after some practice it becomes more natural. 
Find which size works for you, and get comfortable with it! 
Another note about tubing: don’t use the cheapo tubing that is used in commercial drink machines. My friend DJ Mpact found this out the hard way! The cheaper, more flexible vinyl tubing tends to transfer vibrations much more intensely, which could result in an intense headache!

Source:
http://mootbooxle.tumblr.com/post/106170382328/the-talkbox-faq