My Norton is a Mk 1, so kick only; but on electric starter bikes (which is all the rest....) I use 4 gauge. That is almost certainly overkill but as MexicoMike commented, the difference in makes in starting ease and time is remarkable.
I get mine from a place that sells welding cable and terminal ends (plus little solder slugs) so you can make your own to spec length - which is incredibly easy if you have a propane torch, maybe even works with a good heavy duty soldering iron.