This paper describes a computational procedure to determine the minimum ship size to achieve the required seakeeping performance for a given design speed in a specified sea state. The procedure contains four basic elements: a procedure for representing alternative hull form designs with various sizes, a procedure for predicting the seakeeping performance of a given design, a non-linear direct search procedure and a user interface to provide the objectives and the constraints of the problem. The seakeeping performance of the ship in a specified sea area is expressed as a function of the ship size. A non-linear optimization problem is then formulated and solved by using non-linear direct search techniques. The objective is to find the minimum ship size satisfying given seakeeping criteria for a given design speed in a specified sea state. A typical example of determining the minimum dimensions for a patrol boat design is presented. The resultant design is shown to be the smallest vessel that could operate without violating the given set of seakeeping criteria.