The aerodynamic coefficients, forces, sizing parameters, and performance of fixed wing drones in different solar system bodies are investigated. Before carrying out the aerodynamic analysis and study of the performance of fixed wing drones, the physical and atmospheric features of various solar system bodies are first determined. Two drones are considered, namely, a fixed wing micro air vehicle with Inverse Zimmerman planform and a tilt-rotor unmanned air vehicle. The effects of Reynolds number on the performance of these drones from an aerodynamic point of view are determined. Furthermore, a parametric study for sizing of fixed wing drones are also investigated. These analyses ultimately lead to the optimum sizing process of fixed wing drones to be used in solar system bodies with different physical and atmospheric features. The comparison between different atmospheric characteristics on the performance of drones is needed to design efficient fixed wing drones which are capable of flight in other planets.