What gauge size and fuse should I use?
Since I cannot answer in the comments I'll add some little tips.
About the wiring, if you don't want to bother with calculation (usually better but some quick answer might be sufficient), there is some web calculator that I found well designed: circuitcalculator.
When you define your wire size, take around 20% margin of your wire max current as a max fuse.
About your fuse issue on USB socket, well since it is integrated you cannot add fuse between the internal DCDC converter (12V -&gt; 5V) to add the right fuse.  (as @Justme said it have big chances to be a step down converter).
Since you are blind on the efficiency of the converter (it could be LDO or high efficiency converter), You can plus a calibrated resistor that leads you to 1A @5V and check the voltage on 12V line. If it is around 0.5A @12V it means that you have &gt;80% efficiency.
So take the max you think it can handle, calculate the approximate current on 12V you get and maybe take a 20% margin on your fuse choice.
Hope that's helpful