To get a truly accurate result, one would need to know the maximum current drain of the winch and the distance to the battery.
6mm automotive twin( actually 4.5mm2) is too light to run the length of the car and up the drawbar.
I know this first hand, I have a mate that has a winch wired with this, and it is a sad sad situation.
I'd be thinking 10mm2 would be a good place to start......#8 battery B&S is a bit lighter at 7mm2 ish ( possibly a little light), #6B&S at about 13mm2 would be reasoable.
Some of the boat shops are now stocking 10mm2 solar twin.
Think about this......how does your winch cable look beside your trolling motor cable... and you arent trying to yank over a tonne of boat on with your trolling motor.
cheers
Its the details, those little details, that make the difference.