Hope those help.

 

Like it was mentioned the factory uses 4 AWG but you can use as thick a cable as you can work with.

In my truck I have 00 AWG and a group 31 gel cell battery

 

I bought Ford replacements for mine a few years ago. Kept having starting problems in cold weather. Starter and alternator both tested fine. Got to talking to my local parts manager at the time and he suggested the new cables. They had had several do the same thing, The corrosion can't be seen under the coating. Took mine off and bent them and it sounded like I was breaking several wooden sticks. Replaced them and no more problems.