what section are you replacing?
the longer the piece the thicker it should be
30 amp cable is 4mm2 and is not battery cable, if your referring to cables from the battery to chassis, or terminal block then they should be 35mm2 at a bare minimum (thats probably factory size) but i wouldnt use under 50mm2 cable on my own car.
with my battery being in the boot i have the factory 35mm2 cable plus a 70mm2 cable strapped alongside it, with 70mm2 cables between the alternator,starter motor and junction block