Most ppl use either 1/4" or 1/8". I like the 1/8" cuz it's a little easier to route 5 runs of it through the cab. I just ran all of em through a piece of 5/8" line so they wouldn't get kinked or crushed under the floor.
i used 1/4 line, picked it up at napa, they should have it near you, just ask for dot approved 1/4 line. i used 2 dual needle gauges, ill snap a pic when i get home, mounted them in a mounting plate i got from fbi.com