This is perhaps more complicated than it looks.
If you use Xorg without configuration, it should use the native resolution of your monitor. For my laptop: $ xdpyinfo | grep -B2 resolution
screen #0:
dimensions: 1366x768 pixels (361x203 millimeters)
resolution: 96x96 dots per inch
Note that without configuration, Xorg forces dpi=96.
..but my screen is not 361x203mm, it is approximately 276x154mm, which equates to 125dpi (calculated on the screen diagonal).
Edit: this should give the correct monitor dimensions: xrandr | grep "connected pri"
eDP1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 280mm x 160mm