The dingbat called "OrnamentDiamondSolid" is too big. I tried putting footnotesize on it, which helped a little. How can I make it yet smaller?
\documentclass[14pt]{beamer}
\usepackage{bbding}
\begin{document}
\begin{itemize}
\item[\footnotesize{\OrnamentDiamondSolid}] Text
\end{itemize}
\end{document}
\scalebox{0.25}{\OrnamentDiamondSolid}
?\footnotesize
does not take arguments, look at Clara's answer to see how it's supposed to be used.