Nominally you would want 6 volts across the resistor and six volts across the fan motor. However the system will charge somewhere around 14.5 volts. So if you had around 7 volts across the resistor and the motor was drawing 6 amps, you would be dissipating 42 watts through the resistor. So I would fuse it around 10 to 15 amps and a size 14 wire would do the trick.
