This paper presents a numerical study on the hydrodynamics of bottom-hinged plate wave energy converters in regular and irregular waves. A parametric analysis of the plate width and height was performed. Both fully submerged and surface-piercing plates were considered. Two distinct models were developed based on linear hydrodynamics. The first one, in the frequency domain, assumes linear forces. The second one, incorporates fluid viscous and other nonlinear effects. For efficient wave power extraction, fully-submerged plates require amplitudes of motion larger than the surface-piercing ones. Such amplitudes may be unrealistically large close to resonance conditions, which in practice can negatively affect the efficiency. The adjustment of the plate natural period through the modification of the system inertia was tested without any significant improvement in the hydrodynamic efficiency. Resonance design criteria, used in heaving point absorbers, seem to be less effective in this case due to the large viscosity-induced damping and constraints in the plate displacement amplitude. Results show that a hydrodynamic efficient plate should have a width-to-water-depth ratio between 2 and 5, presenting a capture width per unit plate width of approximately 0.8 for regular waves and 0.65 for irregular waves, considering the most usual wave periods.