The influence of geometrical parameters on the electromechanical efficiency of the plasma synthetic jet actuator is investigated with electrical measurements and phase-locked planar particle imaging velocimetry measurements. Two actuators with changeable electrode distance are tested. The first actuator features a fixed cavity volume of 942 mm3; whereas for the second actuator, fixed cavity volume can be adjusted between 50 and 150 mm3. As a result, jet actuator improves significantly with an increasing nondimensional heating volume for both actuators. At the same value of Σ, the efficiency of the small-cavity actuator is approximately 30 times higher than that of the large-cavity actuator which is mainly ascribed to the losses caused by arc-induced shock waves. From the perspective of energy efficiency, the long electrode distance and small cavities in elongated shapes are recommended in the actuator design due to the considerable difficulty in producing long diffused arcs.