The OEM wiring is 4 (might be 6 but I don't think it was) make sure it is OFC (oxygen free copper) and a high strand count (that's the flexibility).
Personally if you are pulling a lot of juice (for a big system, airbag compressors, etc.) I would use 0awg...but if you're going to be staying fairly stock on your electrics it's not that huge a deal...
You could also do a big 3 upgrade while you're at it (engine -> ground, bat -> ground, alt -> bat) to help lower resistance...buts that takes a lot of wire with a trunk battery mount
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