I don't think the wire size really matters. If you're using smaller diameter wire, you might have to up the wire feed a bit. The best thing to do is find some material to practice on (bed sides are 16 gauge) to get your welder set up.
Penetration isn't a real big issue with plug welds. You just need to be sure you don't blow thru the back steel, and get the edges of the hole welded all the way around.
