The dimensioning of the prime mover of a wave energy converter (WEC) is a key step in the entire WEC design process. This paper presents an automatic optimization methodology that focuses on the preliminary dimensioning of a point absorber WEC. By means of the proposed methodology, the engineering problem of dimensioning a WEC becomes a mathematical optimization problem. The methodology is based on a multi-objective differential evolutionary algorithm that optimizes the geometry of a heaving two-body point absorber WEC where the power take-off (PTO), its control, and location are preselected. The algorithm considers two cost functions (WEC surface and annual energy extraction) and several constraints related to PTO characteristics or wave climate. This paper proposes a case study based on a real WEC development project in order to present the methodology. In addition, the impact on the results of the definition of the optimization problem is evaluated. The changes in cost functions and constraints evaluated have led to different geometry solutions. This automatic methodology could be easily integrated in the entire WEC design process, providing a set of properly selected base cases to focus on the detail engineering, among other potential applications.