When I did this mod, I went to Home Depot and brought the breather connector with me. I went over to Plumbing and found some tubing. I was tossing between 1/4" OD x .170 mm (I think) ID and the 5/16" OD x 3/16" ID.
So, I put the connector in the first one (1/4" OD) and saw that it was a very tight fit. The only thing I didn't like was that the walls of the tubing were kinda thin. Then I put the connector in the 5/16" OD tubing. It was a tight fit, but not extremely tight like the first one. The thing I liked about the 5/16" OD was the walls of the tubing were thicker. I figured the tubing is exposed to rough conditions under the truck, so the thicker tubing should take a better beating. In addition to the 5/16" OD tubing, I bought a small clamp for a secure fit.
Well, when I got home, I put the connector back in the tubing just to check the fitting again, and it got stuck! So, I feel I picked the right tubing!
I'm guessing I used 6' of tubing. That sounds like a lot, but it really doesn't look like it. I left enough tubing coiled up and zip-tied underneath the truck so that when I lift it, I can let out some slack.