What size of 12v transformer will run 600watts?

Amps * Volts = Watts

Amps * 12 = 600

600/12 = Amps = 50 amps

You would need a reserve capacity, so I'd go somewhere between 60 or 100 Amp rated transformer.

Transformers are rated in volt-amps which is usually calculated the same as watts. But the term "watts" technically does not apply to transformers. So you need a 600 volt-amp transformer or, as Redbeard has suggested, you need an 800 or 1000 volt-amp transformer.

That's a lot of amps for a 12 volt system so I recommend you double check your requirements. You will need a #2 gauge wire if your requirements are correct.