If you are only starting with it, I would go with a #2. If you are running anything else like a heavier charging circuit, or a duel battery system, with accesories like a winch, definatley go with the biggest you can get. Wire size = amperage capacity. When amperage capacity is at its peak in a wire, it then creates heat from the resistance, think of it like a hose, you have a 3/4" hose at 25 psi, you get X amount of gallons per minute, you have a 1" hose at 25 psi you get X amount of gallons per minute, PSI being voltage and hose size being amps capacity.. Hope this helped