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We have developed prototype resistive plate chambers (RPCs) for the LEPS2 experiment at SPring-8 in Japan. At LEPS2, RPCs will be used as time-of-flight detectors. The goal is to achieve a time resolution of 50 ps with a readout cell as large as possible. We made several types of RPCs and examined the dependence of time resolution with the size and the shape of the readout cell. We found that the time resolution and the efficiency largely depend on the shape of the readout cell and the position where it is excited. We confirmed that two-ends strip-shaped readout cells performed better than one-end readout cells, and a promising time resolution of 60 ps was obtained with a 2.5 cm (wide) × 108 cm (long) readout.