Changing default width of blocks in beamer

You could define your own block environment with an optional parameter for its width and provide a default value such as .9\textwidth.

Here's an example:

\documentclass{beamer}
\usetheme{Warsaw}
\usepackage[english]{babel}

\newenvironment<>{varblock}[2][.9\textwidth]{%
  \setlength{\textwidth}{#1}
  \begin{actionenv}#3%
    \def\insertblocktitle{#2}%
    \par%
    \usebeamertemplate{block begin}}
  {\par%
    \usebeamertemplate{block end}%
  \end{actionenv}}

\begin{document}

\begin{frame}
\begin{block}{Standard}
  Normal block
\end{block}
\begin{varblock}[4cm]{New block}
  Variable width, here 4cm
\end{varblock}
\begin{varblock}{New block}
  If no width was given, .9\textbackslash textwidth will be used
\end{varblock}
\end{frame}

\end{document}

I posted this example here in 2008.

If you do not require a new block environment with customizable width, but simply want to change the width of the original block environments, add the following to your preamble:

\addtobeamertemplate{block begin}{%
  \setlength{\textwidth}{0.9\textwidth}%
}{}

\addtobeamertemplate{block alerted begin}{%
  \setlength{\textwidth}{0.9\textwidth}%
}{}

\addtobeamertemplate{block example begin}{%
  \setlength{\textwidth}{0.9\textwidth}%
}{}

Using the code from Stefan Kottwitz and doing minor changes, I have this in my preamble.

% Variable width block
\newenvironment<>{varblock}[2][0.9\textwidth]{%
    \setlength{\textwidth}{#1}%
    \setlength{\linewidth}{\textwidth}% required to itemize respect the width of block
  \begin{actionenv}#3%
    \def\insertblocktitle{#2}%
    \par%
    \usebeamertemplate{block begin}}
  {\par%
  \usebeamertemplate{block end}%
  \end{actionenv}}

% Variable width example block
\newenvironment<>{varexampleblock}[2][0.9\textwidth]{%
    \setlength{\textwidth}{#1}%
    \setlength{\linewidth}{\textwidth}%
  \begin{actionenv}#3%
    \def\insertblocktitle{#2}%
    \par%
    \setbeamercolor{local structure}{parent=example text}%
    \usebeamertemplate{block example begin}}
  {\par%
  \usebeamertemplate{block example end}%
    \end{actionenv}}

% Variable width alert block
\newenvironment<>{varalertblock}[2][0.9\textwidth]{%
    \setlength{\textwidth}{#1}%
    \setlength{\linewidth}{\textwidth}%
  \begin{actionenv}#3%
    \def\insertblocktitle{#2}%
    \par%
    \setbeamercolor{local structure}{parent=alerted text}%
    \usebeamertemplate{block alerted begin}}
  {\par%
  \usebeamertemplate{block alerted end}%
    \end{actionenv}}