The size and type of motor you need will depend on your robot's weight, how fast you intend it to move, how quickly you want it to accelerate and stop, and how long you want the batteries to last. The heavier the robot, the faster and the quicker to accelerate, the more powerful the motors will need to be. Enigma Industries makes an extremely interesting Drive Train simulator that will let you define your robot's specification, including batteries, motor type, gear ratio, weight. You can then drive your robot in a 3D software simulation and watch its behavior and current consumption as it moves. This is a truly amazing piece of software which will take much of the guesswork out of robot design. Note that the simulator does not yet include a simulation for the controller's features (current limiting, programmable acceleration). A link to this company can be found on the links page.