Hi
A lot depends on what length the cable has to be, what current draw is required to start the engine. Is it an outboard or inboard, petrol, diesel?
What size capacity are the batteries?
Normally I would go as big a size as possible but it can get to silly thickness.
Most cable is give in diameter for thickness, the minimum is what you already have unless it is showing signs of the plastic covering has started to melt!
Sorry I can not be more help at the moment
Coddy