The m2-atx comes with 16awg wires for the power, battery, and accessory wires. This will be fine for standard use, with short runs of wire and the wire in 'open air' (i.e. not encases in spiral wrap or similar). By 'standard use', I mean you giving about 40% overhead on the wattage you use compared to its rated max.
However, if you are going to have long wires runs (i.e. power supply in the trunk), and/or encase the wire in something, and/or use the power supply close to its rated capacity, I would use larger wires. 12awg would be a better option, or better yet 10 awg since it's generally not much more expensive then 12awg. And you can still use common electrical terminals.
If you do use a larger gauge wire, DON'T use the wires included with the m2-atx. For example, don't connect your 12awg wire to the 16awg that comes with it. Instead, connect the 12awg wire all the way to the m2-atx.
Not everyone seams to agree with me when I recommend wire gauges to use around here, although I base recommendations on common standards for wiring, so take my advice however you want =)