The windowsize command adjusts the graphics window to the specified width and height in pixels, thus also setting the default dimensions of subsequently saved images and recorded movies. If only one number is given, only the width will be adjusted. Using windowsize without arguments reports the current pixel dimensions of the window in the status line and in the Log.
Whether to restore the window size of a session is set in the Sessions preferences. See also: save image, movie, view, making images, making movies, ChimeraX window