Objective: To describe the accuracy and precision of a size estimation in wireless capsule endoscopy (WCE) and to develop a simple device that facilitates learning of an exact size estimation in WCE. Design: A prospective study with an animal model. Seventy-five medical students were compared with 21 physicians not performing WCEs and 21 experts actively performing WCEs. Interventions: Video sequences were recorded with a PillCam wireless endoscope in a porcine small-bowel model after implantation of "lesions" of defined sizes. Volunteers estimated the diameter of these "lesions" with and without the use of a calibrator device, which was a picture from a black-and-white grid taken with the PillCam in direct contact with the transparent dome. Results: The students overestimated the diameters of the lesions by 26%. The physicians and experts underestimated the diameters by 32% and 31%, respectively. With the help of the calibrator device, the students underestimated the diameters by 15%. Physicians underestimated the diameters by 21%, whereas experts still underestimated the diameters by 35%. The interindividual log-scale standard deviation (SD) of the estimated diameters from the true diameter decreased during training in all groups, whereas intraindividual SDs decreased in students and increased in experts. Limitation: An animal model. Conclusions: All investigators agree better when a calibrating device was offered. Experts who performed more than 400 WCEs tended to be more precise.