Human symbiosis service robots of various sizes have already been developed. However, few quantitative investigations have been made concerning the influence of the size of a robot on a user's impression. We focused on the height of a robot (robot size), investigating the effect of robot size on the anxiety or threat felt by a human to be caused by a robot and the appropriate human-robot distance. We prepared three mobile robots that were 0.6 m, 1.2 m and 1.8 m tall. One of these robots approached a male subject from a distance of 3 m, at a maximum speed of 0.4 m/s, and the subject stopped the robot using a switch when he began to feel anxious. We measured the distance between the human and the robot when the subject stopped the robot. Then, we asked the subject to complete a questionnaire to evaluate differences in anxiety levels caused by robots of different sizes. As a result of the experiment based on 19 subjects, we were able to observe a tendency for the human-robot distance to increase along with the size of the robot. From the questionnaires, we found that the subjects felt most anxious with the 1.8-m-tall robots, but that some subjects also experienced anxiety with the 0.6-m-tall robots. Considering both the experimental results and the questionnaires, we conclude that 1.2 m is better than other two sizes.