beamer dimendione dei font

ciao a tutti,

vorre farvi una domada.

io l’ultima slide della mia presentazione l’ho fatta così:

`
\begin{frame}

\begin{alertblock}{}

\centering
\textbf{Thank You}

\end{alertblock}

\begin{columns}
\begin{column}{0.5\textwidth}
Any questions?
\end{column}
\begin{column}{0.5\textwidth}
\begin{center}
\includegraphics[width=0.6\textwidth]{./figs/to_think_about.pdf}
\end{center}
\end{column}
\end{columns}

\begin{block}{}
\small \underline{key words}: Cycle disease, waterborne, Operation and Maintainance, Co-digestion, Water supply
\end{block}
\end{frame}

`

io vorrei ridurre la dimensione dei font nell’ultimo block, come vedete ho usato \small ma non ottengo il risultato sperato

grazie 1000 in anticipo

`\documentclass{beamer}

\begin{document}
\begin{frame}
\begin{block}{}
\underline{key words}: Cycle disease, waterborne, Operation and Maintainance, Co-digestion, Water supply

{\small \underline{key words}: Cycle disease, waterborne, Operation and Maintainance, Co-digestion, Water supply}

{\footnotesize \underline{key words}: Cycle disease, waterborne, Operation and Maintainance, Co-digestion, Water supply}
\end{block}
\end{frame}
\end{document}`
Ciao
Antonio