I have to run power to a building 1200 amps service, the building is located 1500ft from FPL transformer the voltage will be 277/480, what size wire should I use.

You should use whatever size wire your foreman or supervisor tells you to use.

1200 Amps 277/480 volt run at 1500 feet?

Even I will not touch this.

Seriously, this needs engineering.

Or bring the high voltage 1450 feet, then change over to 480/277 volt.