It is not worth the trouble to change Emacs for it. But please
test this on another window manager.
I don't have another window manager installed, and if I did, it
would also keep all the windows the same size as the screen, so
someone else will have to test it.
Using sawfish, with a fully maximized Emacs frame, starting with
the font -Sony-Fixed-Medium-R-Normal--24-170-100-100-C-120-ISO8859-1,
set by xrdb-ing a file containing the line you suggested,
(set-frame-font "6x12") sets the font to "6x12" and shrinks the frame
accordingly, whereas (set-frame-font "6x12" t) does the same but keeps
the frame size the same. All exactly as documented.