A hybrid system of a spar-type floating offshore wind turbine and a heaving annular wave energy converter (WEC) provides a promising solution for collocated ocean renewable energy exploitation. The performance of the hybrid system depends on the dimensions of the WEC. Here an optimization method is proposed to determine the outer radius and the draft of the WEC under the wave condition in a randomly chosen operational site. First, three candidate models are selected based on three operational conditions of energy harvest: (1) The natural frequency of the system is matched with the peak wave frequency in the target site (referred to as synchronized mode), where the wind turbine and the WEC nearly heave together in a near-resonance condition, (2) The natural frequency of the WEC is matched with the peak wave frequency (ring mode), (3) The maximum wave power is harnessed under the peak wave frequency (target mode). Then the candidate modes are evaluated to obtain an optimum. Results show that the extracted wave power under the above operational conditions has an upper bound that can hardly be surpassed by enlarging the dimensions of the WEC only. The optimal annual wave energy production is achieved in the synchronized mode because of the superior performance of WEC over a wide bandwidth of effective energy conversion.