This paper presents a strategy for optimizing power switch sizes in DC-DC converters, in order to maximize the power effciency. The proposed strategy is particularly well suited for multi-switch topologies, which have become increasingly widespread in recent years. The effectiveness of the proposed optimization strategy is demonstrated for the case of a 3-level inverting buck-boost converter, which has been studied in detail considering various combinations of input voltage, output voltage and load current. The goodness of the model is verified by comparing the calculated effciency results with the effciency values obtained through Cadence Virtuoso simulations. The derived regression index between the calculated and simulated effciency results exceeds 0.99 in most cases: the proposed model and optimization strategy, therefore, well match the simulation results.