One problem I had when turning a LaTeX Beamer file into a Sweave file was image sizes. When an Sweave file has \documentclass{article}, plots are modestly sized and centered. But when I tried including a plot with an Sweave file with \documentclass{Beamer}, the image was so large that it covered up other material on the slide. The solution was to include the following line immediately after the \begin{document} command:

\setkeys{Gin}{width=0.6\textwidth}

(See section 4.1.2, page 14 of the Sweave manual.) This command made the image the size I wanted, but the image was no longer centered. To center the image, I added \begin{center} before and \end{center} after the Sweave figure command. This worked. A sketch of the code is included below.

\documentclass{Beamer}
\begin{document}
\setkeys{Gin}{width=0.6\textwidth}

\begin{frame}
\frametitle{...}

Slide verbiage ...

\begin{center}
<<&fig=TRUE, echo=FALSE>>=
# R code ...
@
\end{center}