Guage will vary with a variable of things. Length, componets, relays, heat of the compartment.

Things like gauge lights and ignition source indicater wires can use 20 gauge. Water pumps, Fans and fuel pumps, may need 10 gauge or better depending length from power source.

But in most cases 12 gauge is plenty. Instead of having heavy gauge wiring running around everywhere I would use relays for remote locations. If you have a high draw short length wired componet use a HD switch with a matching wire gauge.

Dont rely or trust your typical/elcheapo 20 amp switchs that you might get at your local auto retail shops. They by far dont handle the amps they are advertised at.