The OEM is 4 gauge, I was going to consider 6 gauge, But I am not happy to chance it... Instead I have managed to find a slightly thinner YTX 20 HL battery, that can allow a better gap for the power cable, Which up until then, looked so iffy as it was touching against the inside of the oil tank. (horse shoe type with internal battery box). Which has given me peace of mind at long last... The best thing about it, Is it was half the price of the original one (£51 inc postage) yet still has CCA of 300amps... pretty much the same as the OEM type but 13mm thinner, so nice gap for my cable!If you talking about the cable from the battery to the solenoid, which makes the connection to the starter motor, this ideally needs to be a thick as possible to handle the current!
I believe the oem cables are 6 gauge, so I would suspect that you would not want to go smaller than that!
If replacing it, a good quality, multi stranded cable, the more strands the better for flexibility to resist vibration, cable is the way to go, anything cheap will not perform, get hot and or restrict the current flow and give poor starting.
For reference:- 12 Volt Wiring: Wire Gauge to Amps