We present a novel technique to automatically calculate an initial sizing of analog circuits that conforms to good design practice. The method is purely (DC) simulation-based and does not need symbolic design equations or user design knowledge. It identifies the space of feasible design parameters based on implicit specifications, which arise from the circuit topology. A sizing centered within this space is obtained by iteratively solving a maximum volume ellipsoid problem on approximations to the feasible parameter space. The result is well-suited as initial sizing because it safely satisfies all implicit specifications. Experimental results demonstrate the efficiency and reliability of our method. 1.