What size generator 1200 amps?

First you need to specify what voltage you require the generator to supply...Lets Assume its 240 Volts AC. Watts=Volts x Amps so 240x1200= 28.8 Kw.

Then you have to take the efficiency of the generator into account. Mechanical power in is usually quite a bit more than the electrical power you get out...The difference is mostly the mechanical and thermal losses added together. If we say the generator is 70% efficient then you will need a generator 30% more powerful to supply a constant 1200 Amps...Thats a generator of about 38-40 Kw.