In this paper, we evaluate the size of a mobile robot using the augmented reality (AR). The mobile robot was equipped with a small robot and moved toward a human. Firstly, we made a nested marker that was robust against distance change. Next, we developed a mobile robot with box-like shape. Finally, we made a computer graphic model of a small robot, and rendered it on the mobile robot using AR. We conducted experiments to evaluate three robot sizes. Then we asked participants to answer a questionnaire on psychological threat and visibility. As a result, participants preferred the smallest-sized robot.