For a correct selection of a wire size you need the expected current requirements, acceptable voltage loss or drop and the length of the wire. This way you can be sure that the wire can be trusted to power the attached device without nasty side effects as heat, smoke and fire. Which would end a flight prematurely.
Document below shows the conversion between American Wire Gauge (AWG) and Metric wire sizes.
AWG - Metric conversion, diameter table and wire selection tables
Excerpt from the document:
American Wire Gauge (AWG) sizes may be determined by measuring the diameter of the conductor (the bare wire) with the insulation removed. Refer to the Wire gauge Diameter Table for dimensions. When choosing wire gauge, the distance the wire must run and the amperage it will be expected to carry must be determined first. Refer to the Wire gauge Selection Table. Note that you can always use thicker wire (lower gauge number) than is recommended.
To continue reading see the pdf below: