x-display-mm-height

x-display-mm-height is a built-in function in `C source code'.

(x-display-mm-height &optional TERMINAL)

Return the height in millimeters of the X display TERMINAL.
The optional argument TERMINAL specifies which display to ask about.
TERMINAL should be a terminal object, a frame or a display name (a string).
If omitted or nil, that stands for the selected frame's display.

On "multi-monitor" setups this refers to the height in millimeters for
all physical monitors associated with TERMINAL. To get information
for each physical monitor, use `display-monitor-attributes-list'.