A system for measuring physiologic characteristics or treating tissue in a body lumen. The system may be used for treating abnormal mucosa in the esophagus. The system comprises an energy delivery device, such as an ablation device including an electrode support and an electrode pattern on the support. The ablation device may expand in size from a pre-deployed state to a deployed state. The system may include a sizing device having an inflatable balloon on a distal end of a catheter that is inflated with an expansion medium to expand the balloon to engage the wall of the esophagus so that the internal cross-section can be calculated or measured. The sizing device may also a controller for delivering an expansion medium and measuring the amount and pressure of the expansion medium inside the catheter. Methods of treating or diagnosing tissue are also disclosed.