Well 55 watts×6 lights is 330 watts total. 330 watts/12 volts is 27.5 amps total draw for the 6 lights. I would run 10 gauge wire from the battery to the relay, 14 gauge from the relay to each light, and probably 14 gauge for the relay power switch as well. A 40 amp relay would be good and a matching fuse.
Just as a reference point, my LED strip rock lights I made for $30 or so pull less than 2 amps.