2005 Chevrolet Silverado Crew Cab 1500 V8 Subwoofers And Amp Questions?

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First of all I want to put 2 subwoofers in the truck under the back seats and I heard that the biggest they can be is 12'', is that true? Second, I would prefer not to have to get a new battery or alternator so what is the highest watts that I should having going to the subs and amp? How strong an amp should I get (watts)?

Thanks in advance.