Tag collision is a major problem in the field of multi-tag identification in RFID systems. To solve this problem, many RFID systems adopt their own collision arbitration algorithms based on framed-structure slotted Aloha (FSSA) due to the simplicity of implementation. The frame size, meaning the number of slots in a frame, is a very important factor to inventory tags' responses in the FSSA. How to assign the frame size is therefore crucial to the collision arbitration performance. Since the existing collision arbitration methods do not consider the slot times of each slot when assigning frame size, they may increase overall identification time. By involving the slot times, we improve the collision arbitration performance of the conventional methods. Simulation results show that collision arbitration based on the proposed method is superior to that based on the conventional methods, irrespective of the number of tags.