Diameter of the wheels, speed and torque:

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) :

Increasing the radius of the wheel will increase the velocity of the robot, but also increase the resisting torque that the robot have to overcome. Thus, varying the radius of the wheel, you can make a compromise between linear velocity and torque of the robot.