Recently, it was shown that semiconductor-metal hybrid structures can exhibit a very large magnetoresistance effect, the so-called extraordinary magnetoresistance (EMR) effect. This led to the perspective of using EMR devices in magnetic-field sensors and ultrafast read heads. Based on the finite element method, we study the EMR and optimize the effect with respect to material parameters and geometry. As the important design rule we find that the width-to-length ratio of a rectangular device should be below 0.042. This holds for a broad regime of mobility μ in the semiconductor and specific contact resistance Ï c between the semiconductor and the metal.