The resistance needs to be as low as possible. The bigger the cross section the better because although the galvanic voltages and currents are quite low, cable losses can negate the bonding function.
4mm2 or larger ideal, preferably tinned, with crimped and soldered terminations. Most normal supply wiring cable will not exceed 2.5mm2, and much of it will be 1.5mm2
Technically 8AWG is the smallest of the "battery cable" category, meaning it takes proper sized crimp on lugs.
The next size down, 10AWG would use the yellow crimp connectors. If you look at a yellow ring terminal for a 10 or 12mm bolt, i.e. what you would use on the back of your anode for example, the ring terminal has barely any meat around it and very flimsy. Not the sort of thing you want buried and hidden in the bilges and playing a major part in keeping your boat from sinking....