To promote the commercial application of offshore floating photovoltaic (FPV) systems, an optimization design method for the main geometry parameters of FPV single modules is proposed, which takes the hydrodynamic performance of a single module of FPV array system as constraint and minimizes the construction cost of single modules. An analytical calculation method is established for wave loads on FPV modules based on the Morison equation, and a parameterized evaluation method for the motion response of FPV modules based on motion spectrum analysis is formulated. The hydrodynamic performance of FPV modules of typical size is calculated using DNV Sesam Wadam software to verify the proposed analytical calculation method. Based on the fast analytical calculation of FPV module motion response, an optimization method for the FPV module design with the lowest construction cost is achieved through a direct search method, taking the FPV module size parameters as the parameters to be optimized and under the constraints of the module motion response. With this method, the optimal geometry designs of three FPV modules with different capacities are obtained, and it reveals that larger-scale FPV module is a more favorable option in terms of construction cost per kilowatt-hour and motion response performance. The proposed optimization design method can be used to efficiently carry out the initial design of FPV single modules.