The cable I am using is about 23mm2 and rated at 135 amps.
The shop that supplied the Redarc, said they often use the 6 Gauge cable (rated around 80amps), and is what is usually supplied when you buy the full kit.
I decided to use the 4 gauge as it looks the same size as the starter cable on the vehicle. (and I already had about 4 metres of it)
I was even considering running a 4 gauge jumper wire through a separate high amp isolator switch that would by pass the Redarc and the start assist.
I usually carry some jumper leads that are rated at around 150amps. I just had the concerns about the start assist.