This paper focusses on the design of a robust switching control law for an uncertain discrete-time switched affine system. In order to cope with model uncertainties, a novel control law is introduced and its parameters result from an optimization problem, aiming at reducing the volume of the attractive and invariant set, where the solutions of the closed-loop systems converge to. The design is based on a quadratic Lyapunov function and guarantees global practical stability and robustness with respect to parameter variations. Our method and the associated relaxed control law are then compared with existing conditions from the literature and are validated through numerical examples.