\def\fitbox#1{%
    \font\custfont=phvr scaled 1000%      define a Helvetica Regular font of known size
    \setbox0=\hbox{\custfont #1}%         render our text in that font
    \dimen0=5cm%                          the target width
    \dimen1=\wd0%                         the observed width
    \divide\dimen1 by 1000%               normalize
    \divide\dimen0\dimen1%                dimen0 becomes the font enlargement factor
    \count0=\dimen0%                      convert dimen to a count
    \font\custfont=phvr scaled \count0%   redefine the font
    \hbox{\custfont #1}%                  render the box again, this time for real
}