At least a #1 or #0. Some people recommend 00, but they're a little difficult to get a good crimp on the terminals on the ends. I make mine from arc welder cable which has hundreds of strands of very fine wire and is very flexible. I've also got a swaging tool that I use with a 3-lb. hammer and an anvil to crimp the terminals on both ends. The cable, ring terminals, and battery post terminals are all available on Ebay, or any welding supply can probably make up custom cables in any length and size you need.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!