So far I've found that the thickness of the wire influences the resistance. A thicker wire has less resistance. Also, the length of a wire has a linear correlation with the resistance. 2x length = 2x resistance. So your wire resistance will be length / thickness, but that doesn't explain why the thickness should increase with length. What am I missing?
I can think of two consequences of what I've explained above: first, with a high wire resistance, your voltage will drop and you might not get the voltage you need. Second, having a thin wire with lots of amps will potentially melt the isolation and be a hazard.
So let's say we would accept any voltage loss and just don't want a hazardous situation. We take a wire of 1m. Then we run a certain amount of current though it which will produce heat in the wire. Now we double the wire length to 2m, so double the resistance. We run the same current through it which will produce double the amount of heat, but over double the amount of length. Unless you're coiling your wire I don't see how it should make for a more hazardous situation.
And the other way around, according to this calculator http://www.solar-wind.co.uk/cable-sizing-DC-cables.html if you have a 1cm wire and running 100 amps @ 12 volt you can do with 0.5 mm2. How is that possible?