Determine text width

From http://www.latex-community.org/forum/viewtopic.php?f=5&t=2712

Using \the\textwidth will allow you to see the value LaTeX is using. However, the default unit of measurement is probably points, not a likely option for your graphics software.

Fortunately, the conversion function is available in the standard layouts package: \usepackage{layouts}
...
textwidth in cm: \printinunitsof{cm}\prntlen{\textwidth}
will print the value in cm. All the usual LaTeX units of measurement are also available if cm aren't what you want.

The printlen package:

\documentclass{article}
\usepackage{printlen}
\begin{document}
\printlength\textwidth
\end{document}

produces "345.0pt". It can print in units other than pt, too.