In terms of aerodynamics, we advise you to use 21-23c. Because of the wide section of the rim, the tire will be 2mm wider than what the tire is specified. Anything bigger than 25mm (23c tire) will negatively interfere with the airflow. In terms of comfort, you could go for larger than 23c. Especially if you anticipate the surface to be rough. It would allow you to ride with a slightly lower tire pressure.