x-display-pixel-width

x-display-pixel-width is a built-in function in `C source code'.

(x-display-pixel-width &optional TERMINAL)

Return the width in pixels 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 pixel width for all
physical monitors associated with TERMINAL. To get information for
each physical monitor, use `display-monitor-attributes-list'.