The cusped field thruster (CFT) is a class of advanced electric propulsion (EP) technology for satellite and space missions, offering advantages over other types of EP including enhanced electron confinement owing to the magnetic mirror and reduced particle loss effects at the dielectric wall. The increasing demand for downscaling for micro-satellite class platforms while keeping performance at similar level has led to considerable efforts dedicated to physical modeling and performance characterization of downsized CFT. Multi-objective design optimization is conducted in this study by employing performance parameters of downscaled CFT, namely, thrust, total efficiency, and specific impulse as the objective functions to maximize and design parameters including anode voltage and current, mass flow rate, and inner and outer magnet radii as the decision variables. Two geometric configurations are considered, i.e., those comprising three magnets with fixed thickness and four magnets with variable thickness to gain insights into the influence of magnet thickness on the performance. Considerable effects of magnet thickness on the performance have been found, including thrust increase of up to approximately 20% and increase in specific impulse by up to approximately 10%, as compared to the configuration with fixed thickness magnets.