How to use Xresources to specify a frame's pixel size?

[Nick Helm <nick@tenpoint.co.nz>]

Hi all,

I recently read about a change to frame parameter syntax in Emacs 25,
specifically one solution to Bug#21415
(https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21415), which introduced
this syntax:

  (width  . (text-pixels . <width>))
  (height . (text-pixels . <height>))

to specify a pixel width and height in frame parameter commands. 

Other than not finding the syntax documented in the Emacs or Elisp
manuals, this seems to work well. 

However, I like to use Xresources to specify the size of my initial
Emacs frame – typically by setting a character width x height with
something like:

  "Emacs.geometry" = 200x50;

in my local org.gnu.emacs.plist file.

Can I do something similar to specify a pixel width x height for my
initial Emacs frame? If so, how?

Note that the property list key-value pairs used on macOS seem to
closely align with the documented Xresource class-resource pairs, so a
solution on any platform would point me in the right direction.

Thanks.

--
Nick