The function
screen-internal-geometry
returns the geometry (as multiple values representing a screen rectangle) of the region of the screen that can be used to display windows without obstruction. This region excludes "system areas" like menubar and taskbar and so on. Examples of these include the Windows taskbar, the Mac OS X menu bar and the Mac OS X dock.
x and y are the screen rectangle's coordinates as offsets from the top-left of the primary monitor, and width and height are its dimensions.
On Microsoft Windows
screen-internal-geometry
works with document-container, returning the current size of the container (which may vary over time).
screen-internal-geometry
changes.CAPI Reference Manual - 15 Dec 2011