How do i know what size power wire to use in my car for my new radio i plan to hook up two 1200 watt amps an equalizer and a 15 in subwoofer to a 2007 chevy cobalt

Put a generator in the trunk. I am assuming this is RMS watts. This will require modification to the vehicle's electrical system. Lets look at what you are saying. You are going to use two 1200 watt amplifiers, for a total of 2400 RMS watts.

Using a simple rule of thumb based on ohm's law and assuming a little over 80% efficiency in the amps (I doubt it) you divide the total watts by 10 (the cars 12 volts – a fuge), that leaves you a value of 240 or 240 amps required to power the amps at 12 volts DC.

Now consider 20 feet of wire to the battery or forty feet of wire round trip, one for positive and one for negative. a #2 AGW wire, about the size of your finger, will give you a 1.8 volt drop or 7% voltage drop in the wire. Assuming the alternator is maintaining 13.8 (standard charging voltage) that leaves you exactly 12V at the amplifier. This sounds great but the larger alternators only put out about 100 amps so you need 2.4 alternators to supply the current the amps will need. Lets assume a standard car battery is rated at 50 amp hours, that indicates that the battery can supply about 12 minutes of power at full output. Now calculate the rest of the power requirements of the vehicle and what you are short in alternator capacity the battery makes up. I would guess you vehicle battery would be dead in less then ½ hour or you need to keep the volume way down. That indicates a much smaller amp would be a much more cost effective solution.
Good Luck,
Gilshultz