A method is provided that includes surgically implanting an electrode device having a cuff and one or more electrodes mounted in the cuff, by placing the electrode device around a nerve such that the electrodes face the nerve. Conductive solution is introduced into the electrode device such that the solution is in contact with both the electrodes and the nerve. During implantation of the electrode device, an impedance is measured between one or more of the electrodes and an electrical contact point in electrical communication with the electrode device. Responsively to the impedance measurement, it is determined whether the electrode device is correctly sized for the nerve, and, if not, the electrode device is removed and another electrode device having a different size is implanted. Other embodiments are also described.