There are multiple fuses in your car protecting various loads. Wire gauge depends on load. The gauge or wire for the battery might be 8 AWG to handle high cranking currents for starter motor. 14 AWG wire might be employed for loads requiring 15 amps or less and so forth.
Use 8 gauge wire.
Yes, no problem at all going to a larger ampacity of wire. Larger size wire yes, smaller size wire no.
The correct size fuse needed is based on the size wire used in the circuit. The size wire needed is based on the load that will be applied to the circuit. 1. AWG 14/2 wire = 15 amp fuse 2. AWG 12/2 wire = 20 amp fuse 3. AWG 10/2 wire = 30 amp fuse 4. AWG 8/2 wire = 40 amp fuse
Assuming the wiring is sized for 12 amps, you can replace your fuse with any 12 amp fuse or smaller and with a voltage rating at or above what you expect to connect to it. The amp rating protects the wire, so you cannot go above what the wire can handle. The voltage rating is the max voltage that it can safely protect, so you cannot use a fuse with a lower voltage rating than you expect to connect to.
# 3 gauge
4 copper
6 AWG
As with any electrical installation, the wire size used depends on the expected maximum current the wire will carry. That cannot be determined by voltage alone.
4800
The wire size depends on how much current it will conduct.
A fuse rated for 250 v is fine for 240 v. The fuse might be a cartridge fuse, so it must fit the available socket, or it might be fuse wire that has to be fitted in the holder correctly.
Use a 20 amp fuse or breaker.