Without knowing how much power or speakers you'll be running, it's hard to say. I'd run 0-gauge for the main positive and grounds, though -- if nothing else it will give you room to grow and make sure that you never run into issues in the future.
You'll want to size your main fuse block to aproximately 10% of the size of your total max wattage. Example... if you have 2000 total watts, use a 200amp master fuse. Use the same math for the amp fuses (if you're going to fuse them individually, too.)
You'll also want to get some heavy duty battery terminals that can accomodate 0-gauge wire. Keep adding additional terminals if you're using multiple batteries, perk switch or isolator/combiner.
I'd use 12 gauge for the sub. I'd use 12 - 14 gauge for the rest of the speakers.... again.... I'm sure you'd like to leave room for the future.
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.