The range of speed required for this robot is from 0.25 to 0.35m/s. We can take the maximum speed of this robot as 0.35 m/s for the design. Take the diameter of the wheel as 9 cm, because according to the requirement, the ground clearance should be greater than 3 cm and we will fix the robot body in same level as the motor shaft. In that case, we will get more ground clearance.
Using the following equation, we can calculate the RPM of the motors:
RPM = ((60 * Speed /(3.14 * Diameter of Wheel)
RPM = (60 * 0.35)/(3.14 * 0.09) = 21 / 0.2826 = 74 RPM
You can also take a look at http://www.robotshop.com/blog/en/vehicle-speed-rpm-and-wheel-diameter-finder-9786 for the computation.
The calculated RPM with a 9 cm diameter wheel and 0.35 m/s speed is 74 RPM. We can consider 80 RPM as the standard value.