you don't determine wire size by wattage - the only way to choose wire is by looking at the total current draw you will have, the total lenght of the wire, and the acceptable voltage drop across the wire.

One safe rule of thumb is to add up the fusing on the side of your amplifiers an use that as your total current draw. For most 1000W systems, you will be around 120A. Over a 12-14ft run, you are right on the border of 4ga. You typically won't see peak current draw when listening to music anyway (especially if one amp is for highs).

Run a 4ga to the rear, split with a fused distro block and run 8ga to the amps.