You can basically use \setbeamersize{text margin left=1em,text margin right=1em} to set the margins on the left and right. Just change the 1em to whatever margin you like.
\documentclass{beamer}
\usetheme{Stockton}
\usepackage{lipsum}
\setbeamersize{text margin left=1em,text margin right=1em}
\begin{document}
\begin{frame}
\frametitle{Test}
\begin{itemize}
\item \lipsum[4]
\item \lipsum[4]
\end{itemize}
\end{frame}
\end{document}

But please do not use that much text on a slide, as percusse mentioned...
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