A new, general methodology for the sizing of fuel cell-based hybrid-electric aircraft powertrain is illustrated. The method is based on an accurate physical model of the fuel cell module, integrated within a procedure that, given aircraft and mission parameters, estimates the corresponding sizing and performance. The method is validated vs. a real flying prototype, the Hy4. This can be used in the design of conversions of existing aircraft to battery and fuel-cell powertrains, as well as in the preliminary sizing of new propeller-driven air vehicles of arbitrary weight category. The results of the powertrain sizing for the conversion of two existing aircraft in the General Aviation category are presented, in connection to realistic values of the performance of electric motor, fuel cell, battery, and storage technologies involved.