display-mm-dimensions-alist is a variable defined in `frame.el'.
Its value is

Alist for specifying screen dimensions in millimeters.
The functions `display-mm-height' and `display-mm-width' consult
this list before asking the system.

Each element has the form (DISPLAY . (WIDTH . HEIGHT)), e.g.
(":0.0" . (287 . 215)).

If `display' is t, it specifies dimensions for all graphical displays
not explicitly specified.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.