It would depend on how well insulated your system is; the less energy lost, the less energy you need to collect with your mirror to boil the water, therefore the smaller collection mirror needed.

Then you have to look up turbine generator set efficiency to know how much steam you are going to need to turn the generator rotor within the stator field.

And if it is wet steam, it will either rust your turbine out or rip it apart depending on how fast it turns. Another reason insulation is important. The steam has to remain superheated all the way to the turbine.