bug#67449: Manual: add how to change default global font size in .emacs

[Eli Zaretskii]

> From: Kévin Le Gouguec <kevin.legouguec@gmail.com>
> Cc: Dan Jacobson <jidanni@jidanni.org>,  67449@debbugs.gnu.org
> Date: Thu, 30 Nov 2023 08:23:00 +0100
> 
> > just specify the same font
> 
> but I can imagine wanting to keep the system's default _family_, yet
> have a consistent font _size_ across all my setups, in which case having
> to spell out the font in my dotfiles is a bother.

I don't see why it should be a bother.  That same node of the manual
says a bit later:

     To check what font you're currently using, the ‘C-u C-x =’ command
  can be helpful.  It describes the character at point, and names the font
  that it's rendered in.