This paper presents a real time collision avoidance method considering robot's size for an autonomous omni-directional mobile robot by simultaneous control of translational and rotational motions. This method enables the robot, which the size of width is not equal to that of length, to avoid unknown obstacles safely and efficiently. To consider the difference between the width and length of the robot, the Capsule Case is newly introduced. In order to achieve the simultaneous control, this method employs an omni-directional platform. As an example of the design of proposed method, novel control method based on the Fuzzy Potential Method (FPM) is proposed. To test the effectiveness of the proposed method, several numerical simulations are carried out.