Here are some maximums I've found:

8AWG: 650W @ 12V (aka car off), 800W @ 14.4V (avg. for car running)

4AWG: 1,150W @12V, 1,1375W @ 14.4V

It's safe to say that 800W and 1,375W, respectively, are the maximums because no one in their right mind is going to be utilizing their systems peak power with their car off for very long. Keep in mind this doesn't take into account the voltage drop over the 20-25 ft. of wiring. So for those of you thinking it's cool to waste money (with the price per lb. of copper these days) buying 0AWG wire, save it for the big guys. That stuff can handle up to 2,500W.

 

You should pick your wire by the amp draw and not the power of your amps.

If youre drawing 80 amps or less, 4 gauge is fine. Anything over that and you need 2 or bigger.

 

Those sizes I listed were based upon amp draw, but I converted them to wattage based on their respective voltages (12 and 14.4). 8 gauge is good to about 55A and 4 gauge to about 95A, again without voltage drops.