display-mm-width

display-mm-width is a compiled Lisp function in `frame.el'.

(display-mm-width &optional DISPLAY)

Return the width of DISPLAY's screen in millimeters.
If the information is unavailable, this function returns nil.
DISPLAY can be a display name or a frame.
If DISPLAY is omitted or nil, it defaults to the selected frame's display.

You can override what the system thinks the result should be by
adding an entry to `display-mm-dimensions-alist'.

For graphical terminals, note that on "multi-monitor" setups this
refers to the width in millimeters for all physical monitors
associated with DISPLAY. To get information for each physical
monitor, use `display-monitor-attributes-list'.