One of the most important decisions for shipowners is to determine how big a containership to order. The optimum containership represents a trade-off between the cost and revenue resulting from ship size and speed. In reality, there is a tendency toward increasing containership size and speed, which is resulted from some factors related to profit. However, most of the past studies devoted their attention to the problem only from the cost perspective. These models neglected the effect of ship speed on profit, and might result in inadequate solutions to the problems. Based on the cost-volume-profit (CVP) analysis, this paper tries to formulate a model seeking the optimal size and speed of containerships that can maximize the profit for shipowners. The objective function is a strictly concave function, with two decision variables of ship size and ship speed, and its solution is globally optimal and unique. An example of the Trans-Pacific (Fareast-Northern America) Route is employed to test the model formulation and sensitivity analysis is conducted. The results would provide shipowners with a beneficial reference for planning the size and the speed of their containerships.