14 or 16 gauge will be plenty- - - -there's very low current in the primary ignition circuit, less than 5 amps. Since the battery voltage can drop to just above 9 volts during cranking, the bypass wire allows the coil to get enough voltage to maintain a hot spark with the starter running. Otherwise, the resistor would reduce the voltage applied to the coil to 6 or 7 volts, not enough to get a good spark. As soon as the starter switch disengages, the resistor goes back to protecting the points from excessive primary current.
Jerry
