Another method is to observe the constrictions of the stereo pyramid.   Earlier, I mentioned that the stereo pyramid was closely related to the stereo window.  A description of the stereo pyramid may help to make this more clear.
 
The stereo pyramid is formed from two nearly superimposed pyramids, created by the sightlines from the two eyes of any observer and the four corners of the screen or the image.   It actually extends beyond the screen to the furthest parts of the scene that can be visualized.  A screen might be placed anywhere along this pyramid, intersecting with the images, and forming an infinite number of possible positions for the stereo window.  
 
From this, it may be seen that as an object appears to approach the observer in the theater, the closer it comes, the smaller it must be to avoid crossing the edges of the frame, or the edges of the stereo window, or the sides of the stereo pyramid.   If it does cross any of the four edges at any time, this constitutes what is known as a "window violation" and results in an undesirable effect.  Since the stereoscopic images are generated by the lateral deviation, the negative effect of such a window violation is usually more severe on the left and right sides, than on the top and the bottom.   For this reason, wider aspect ratios are usually easier to work with when avoiding these violations.