Can I choose the size of the technology (in nm)? Alternatively, how can I calculate how much power the system will consume if it was made in a different technology?
Different device families are what correspond (mostly) to different technology sizes.
For power measurements/estimations for a design, download and use the appropriate Early Power Estimator (EPE) spreadsheet for a particular device family:
In FPGA scenario, the logic utilization and application matters more than nm and for the power consumption can be calculated with respect to the specific FPGA device in use . Early Power Estimator provides a best way to calculate the power for the particular FPGA .