Changing font size of selected slides in beamer

You can use \fontsize:

\fontsize{<font size>}{<value for \baselineskip>}\selectfont

For example,

\fontsize{6pt}{7.2}\selectfont

changes the font size to 6 points and the \baselineskip to 7.2 points. You can define a new command to apply the change:

\documentclass{beamer}
\usepackage{lipsum}

\newcommand\Fontvi{\fontsize{6}{7.2}\selectfont}

\begin{document}

\begin{frame}
\frametitle{Frame with reduced font size}
\Fontvi
\lipsum[1]
\end{frame}

\begin{frame}
\frametitle{Frame with regular font size}
\lipsum[1]
\end{frame}

\end{document}

Since Gonzalo's answer does not work for nested itemizes, here is a solution redefining the beamer template which solves that.

To make it general I used the environ package, because the normal newenvironment has problems with seperating braces by the body.

\documentclass{beamer}
\usepackage{environ}

\usepackage{lipsum}


%
% Custom font for a frame.
%
\newcommand{\customframefont}[1]{
\setbeamertemplate{itemize/enumerate body begin}{#1}
\setbeamertemplate{itemize/enumerate subbody begin}{#1}
}

\NewEnviron{framefont}[1]{
\customframefont{#1} % for itemize/enumerate
{#1 % For the text outside itemize/enumerate
\BODY
}
\customframefont{\normalsize}
}

\begin{document}

    \begin{framefont}{\tiny}
        \begin{frame}
            \frametitle{Frame with reduced font size}
            \lipsum[66]
            \begin{itemize}
                \item Nunc sed pede. Praesent vitae lectus.
                \item Nunc sed pede. Praesent vitae lectus.
            \end{itemize}
        \end{frame}
    \end{framefont}

    \begin{frame}
        \frametitle{Frame with regular font size}
        \lipsum[66]
        \begin{itemize}
            \item Nunc sed pede. Praesent vitae lectus.
            \item Nunc sed pede. Praesent vitae lectus.
        \end{itemize}
    \end{frame}

\end{document}