I ma glad you are enjoying using Cuety.
About your request, you can edit the size of the effect in the movement effect window, but it will affect the X and Y size at the same time.
To set the effect only on your stage the best solution is to set the position of the moving toward the stage without any fx on. Once your movings are pointing at the center of the zone you want the effect to be displayed you can go in the fx window and set your effect. This way it will be centered around the position the heads were pointing at.
I hope this will help you,
Enjoy