1/4" (rubber fuel line)
Best to buy extra if you plan on running it to the engine compartment. I needed 13' since I left plenty of slack at the axle, and looped the fuel line in the engine bay (to minimize any potential for water intake). Also added a Fram G1 fuel filter to the end of the hose to maintain zero pressure in the diff.
I actually just did mine two weeks ago when I changed the diff's, tranny, and Xfer case fluid to amsoil, I have been thinking about it for a year and just never got around to it. Those parts worked fine.
Just for reference the union was $6.92 and the breather plug was $3.34.