A system for monitoring ablation size is provided. The system includes a power source including a microprocessor for executing one or more control algorithms. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. A plurality of spaced-apart electrodes is operably disposed along a length of the microwave antenna. The plurality of spaced-apart electrodes is disposed in electrical communication with one another and each of the plurality of spaced-apart electrodes has a threshold impedance associated therewith corresponding to the radius of the ablation zone.