With the motor giving constant RPM the wheel diameter is the last criterion to maintain torque- speed balance.
Let V be the linear speed, T be the torque given by the motor, w be the angular velocity
Let Tf be the torque developed by the ground as reaction to the wheel's movement.
This is the torque resisting the robot's movement.
For the robot to move from rest (or to accelerate), T must be bigger than Tf.
The toque Tf = F.r .......(1)
Also note the that the linear velocity of the robot equals the linear velocity of any given point on the outer circumference of the wheel,
Then the velocity V = w.r .......(2)
From the relations (1) and (2) :