With the caution that auto manufacturers don't like to waste money on unnecessary copper so the the OEM cables on 12v installations are often the smallest that will do the job, most of the time. So if you are replacing cables on an 9-2-8n converted to 12V you don't have to use big 6v cables, but something larger than the minimum size Fred's Home and Auto is selling might not a bad idea.
I used to do oversized battery cable installations for Piper aircraft. These had the battery in the back seat, and used aluminum cables. Going a size or two larger, and using copper instead of aluminum, would totally transform cold weather starting with no other changes.