What size electrical wire for 60 amps 210 feet?

4 AWG is the initial answer without considering the length. For 210 feet of 4 AWG the resistance is .052 ohms. So using Ohm's Law the voltage drop would be 60 x .052 = 10.92 volts. Usually the drop should be less than 10 % of the supply voltage. So if the supply was 120 volts you would be allowed 12 volts and for 240 volts 24 volts of a drop.

So you be okay for using either 120 or 240 volts and 4 AWG wold support the current and the length. There may be other factors in your application, but from what you specified use 4 AWG.