There are some truly impressive LaTeX solutions for doing PowerPoint-style font-resizing to fit into a fixed width box. I recently had need of something more simple: print text on one line only, scaling the size down instead of allowing it to wrap. The following LaTeX snippet does exactly this, triggered only if the font width (before wrapping) exceeds \textwidth.

{
  \def\formattedtext{The no-wrap text to scale}%
  \newdimen{\namewidth}%
  \setlength{\namewidth}{\widthof{\formattedtext}}%
  \ifthenelse{\lengthtest{\namewidth < \textwidth}}%
  {\formattedtext}% do nothing if shorter than text width
  {\resizebox{\textwidth}{!}{\formattedtext}}% scale down
}

This requires

\usepackage{xifthen}
\usepackage{graphicx}

to handle the \ifthenelse, \lengthtest, and \resizebox statements.

It works like you might expect: check the width of the text, and then use \resizebox to scale it down, if needed. Such logic isn’t always obvious in LaTeX: arbitrary defs cannot store length information, so you have to set the type of the \namewidth variable as a dimension before you can assign/test it as a length.

As with most helpful things in LaTeX, we can wrap it up in a reusable macro:

\newcommand{\oneline}[1]{%
  \newdimen{\namewidth}%
  \setlength{\namewidth}{\widthof{#1}}%
  \ifthenelse{\lengthtest{\namewidth < \textwidth}}%
  {#1}%
  {\resizebox{\textwidth}{!}{#1}}%
}

which allows

\oneline{\Huge{The no-wrap text to scale}}

\oneline{\Huge{The quick brown fox jumped over the lazy dog, over and over and over and over again.}}

On any reasonable-sized page width, these two lines will not wrap, but the longer line will be stretched horizontally to fit in the available space.

You can find a fully-working (as of TeXLive 2012) gist here.