The paper reports a new methodology for optimal sizing of converters. Considered magnitudes in the design process are: efficiency value, ripple values, constraints for ensuring continuous conduction mode and continuous voltage mode and dynamic requirements about the pole and zero location. The optimization model becomes a Geometric Program because of the posynomial form of the involved expressions. The reported approach allows us to automatize the design of the converter. Concretely, we describe the proposed methodology for a Cuk converter. A design example is showed to illustrate the procedure.