Choosing wire size is an important factor in a system. This size depends on the amount of current passing through the wire. AWG (American Wire Gauge) and metric gauge are two regular scales which are currently used.
In the metric gauge scale, the gauge is 10 times the diameter in millimeters. The higher the gauge, the thicker the wire. For example, a 20 gauge metric wire is 2 millimeter in diameter.
In the AWG scale, the thickness of the wire decreases with increasing gauge size. For example, 10 gauge wire is thicker than 12 gauge wire. You can get the size of the wire for your system depending on amount of the current (amps) from the table below.