If you don't have points anymore, and you are using a ford fender mount style solenoid to power the starter, there will be only 3 terminals used on the ford solenoid. A big one to the battery, a big one to the starter, and a little one to activate the solenoid. That little one can be 18 or even 22 gauge, it only draws 1.5 amps. The big ones can be anywhere from #4 to #00, like others have said, depending on the length and the load. If weight matters, use the thinnest that will do the job.

Edit: SO you are using the ign switch to activate the ford solenoid and the power from the ford solenoid to the mopar starter right? SO you can use 18 g from the ign switch to the ford solenoid. A fat cable from the ford solenoid to the big stud on the mopar starter. Run a 14 g jumper wire from the big 12V stud on the starter to the little stud on the mopar starter.