I am going to fit a 12,000lb 4WD winch to my boat trailer and power it from the car batteries when retrieving the boat which weighs 4,500Lb. The Amp draw for the winch is:
Line Pull (lbs./ kg] Line Speed FPM(m/min) Motor (Amps)
0 33.3 ft (10.3m) 85
4000 (1814) 16.8ft (5.1m) 158
6000 (2722) 12.8ft (2.62m) 231
8000 (3629) 10.1ft (3.1m) 302
10000 (4536) 8.8ft (2.7 m) 371
12000 (5443) 7.6ft (2.3 m) 450
12300(5590) 0ft(0m) 200

What size cable should I use?

Have you tried contacting the manufacturer?

Yes. They only want to discuss the fitting of the winch to the bull bar and therefore cable only needs to be about 1 meter long. I intend to mount the winch on the boat trailer and plug it into a 175 Amp Anderson plug with power running from the front of the car, about 6 meters. I understand I need to allow for voltage drop over that distance henc my question here.

If any body is out there I'm still looking for an answer to my question

Try asking your question in "Electrical and Lighting."