hillbillytim,
It's great to hear you are going to rewire your truck...it's pretty easy and when you are done, you will know your electrical system by heart.
Wire size is based on ampacity. This means a certain size wire will only carry so many amps. Sort of like a small garden hose for the front lawn, but a huge hose for the big garden out back.
I have enclosed a link that shows average (adequate) wire sizes for automotive devices at 12 volts. They tell you how to size up if you want to run 6 volts. Good luck!
Stuart
http://www.crowbarelectricalparts.com/specifications.htm#Gauge
