This function will use the width and height in millimeter units of the plot on which it is invoked as the default width and height for plots generated hereafter.
This function will be active and not grayed out in the right-click menu of a plot whose width and height are different from the default ones.
If you would like to set the default width and height of a trellis plot, you'll have to use Set as Default Trellis Page Size.