you need to tune the number of steps per unit on the motor tunning window.
This number of steps per unit depends on the gearing of the motor, how transmition is done for movement, the encoder resolution (in case of servo) ...
If you are getting a bigger image as you would expecting to have, then put more steps per unit that you have right now.
In fact if you are getting, lets say 100% more big than expected (the double), then multiply the current steps per unit by 2.
Hope that helps