Wiring dimension is most important when dealing with low volt systems. The lower the voltage, the higher the current, and high current in a thin wire means loss. 

For instance, in many systems, the regulator is mounted under the saddle, from here you have a wire going to the ammeter in the front light shell, and back to the battery under the saddle, this means 1,5 m (5 ft) of wire, which can easily give a loss at 1,5 volt. The regulator is adjusted to give 7,5 volt. 7,5 minus 1,5 gives you 6 volt, and not enough to charge the battery fully  

Below you can see which dimension you at least have to use.

The chassis connections have to be of the same sice, including the battery connection


If you have the effect (power), and the distance, draw a straight line and you will see haw thick the wire have to be

Ohms law says: current X Voltage = Watt


Power i watt                         Squere mm                      Length
 24 / 12 / 6v