Good cables are #4 copper.

Then, there is everything else.

If the cables are actually being used to jump - that is supply 100% of the current needed to turn the starter - do not go much below #6 wire, or you will have a melted mess on your hands. If you are simply putting a charge on the existing battery so that it will carry most of the load, then pretty much anything will do as a factor of size and time. A piece of 16 gauge Zip and 20 minutes will give some sort of charge, and so forth.

Peter Wieck
Melrose Park, PA