To choose an adequate wire gauge, determine the amp draw (amperage) that the wire circuit will carry. Then measure the distance that the wire will travel (length) including the length of the return to ground (the ground wire running to the chassis or back to a ground block or battery. Using these two numbers, Amps and length, locate the nearest gauge value in chart below. For 6 volt automotive systems typically a wire gauge 2 sizes larger than what is shown should be used.
When wiring anything in your vehicle that draws heavy current such as high powered offroad lights or audio amplifiers, there are a few things to consider. Number one, make sure you use wire that is rated for the amperage that the accessories is going to pull. It is always better to have wire that is OVER rated rather than wire that is not rated high enough. If wire is used that is not rated to handle the current that your accessory will pull, the result could be overheated wires that could melt the insulation or the electrical plugs found throughout your vehicle (see image below), causing a short or worse yet it could result in a fire. If you know how much current your accessory will draw you can determine what gauge wire is appropriate for your application.
Personally I like to use wire that far exceeds the current draw of my accessory. It's overkill but in a few applications I've used heavy gauge stranded industrial wire with water and chemical resistant insulation. That way there is no question as to whether the wire is rated high enough or not. If this approach is taken, it is very wise to place a fuse at the battery end as close to the battery as possible. Most wire in a vehicle, if shorted out, will burn up before the battery overheats and possibly explodes. If wire that is over-rated for vehicle use is used and a short occurs, a short will most likely result in damage to the vehicle of some sort unless a fuse is put in line as close to the battery as possible. With the fuse there, in the case of a dead short, the fuse will burn out first before any damage could occur.