Well, my ground from the battery to the block was 1 gauge and 7 feet long. And I think that the main power feed was 0 or 0/2. But when the Previous owner installed a perko switch he put 4 gauge between each of the batteries and the switch and also between the two batteries.I also had 4 gauge from the starter relay to the starter solenoid. I did notice some rust/corrosion on the neg terminal that went to the block, so upsizing that and shortening the run by a foot (because I could) made sense.
I think I have the "newer" style starter with the external solenoid (it's not an old school 1 wire starter).
My problems could have simply been the relay. Maybe extra current from the smaller wires and/or some corrosion. I just figured I'd upgrade and upsize because I can and because I don't want to worry about this crap ever again.