I don't believe that the type of motor has much to do with the area you're trying to cover. As long as the torque of the motor will move what ever you're moving at what speed you need to move it, then it doesn't matter if you use NEMA 17 or NEMA 23. If you don't really know about the torque you'll need, then I'd look at the motors they're using for the Shapeoko 3 or similar and go from there. Those motors are probably more than you'll need for pen plotting I would think.