To change the font for the standard \verb command (and verbatim environment) you can redefine \verbatim@font as I did in my example code below; however, since you are already using the listings package, you can use \lstinline for short texts and the lstlisting environment for longer verbatim texts and use \lstset to give the same appearance to both short and long texts.
\documentclass{beamer}
\usetheme{CambridgeUS}
\usepackage{listings}
\lstset{basicstyle=\ttfamily\footnotesize}
\makeatletter
\def\verbatim@font{\footnotesize\ttfamily}
\makeatother
\setbeamertemplate{itemize items}[square]
\setbeamercolor{itemize item}{fg=red!60!black}
\begin{document}
\begin{frame}[fragile]
\begin{itemize}
\item \lstinline{amsmath}, \lstinline{amssymb} and \lstinline{amsthm} for mathematics.
\item \lstinline{geometry} for easy management of document margins.
\item \verb+graphicx+ for managing external images.
\end{itemize}
\end{frame}
\end{document}

\verboutput. – percusse Jun 13 '12 at 23:44