Change \textwidth and \textheight in mid-document

You can use \newgeometry and \restoregeometry from the geometry package:

\documentclass{article}
\usepackage[textwidth=5cm,textheight=5cm]{geometry}
\usepackage{lipsum}
\begin{document}

\lipsum[1-2]
\newgeometry{textwidth=10cm,textheight=10cm}
\lipsum[1-4]
\restoregeometry
\lipsum[1-2]

\end{document}

To change textwidth locally for a defined scope you may use \begin{minipage}{WIDTH}.

\begin{minipage}{10cm}
...
\end{minipage}