The pocket-size Millermatic Calculator recommends .035" or .045" MIG wire for 1/4" thickness. It also recommends .045" or 1/16" FCAW wire for the same thickness. I would go with the smaller diameter but wire speed and voltage depends on whether you desire short circuit or spray transfer. With .035" MIG the calculator suggests 360-380, 180-190 amps, 21-22 volts, C25 shielding for short circuit or 400-420 IPM, 200-210 amps, 24-25 volts and Ar98-2%Oxygen shielding for spray.