Beamer change frametitle font size just for one slide

The {} will ensure, that the change is only done for the slides within the {}.

\documentclass{beamer}

\begin{document}

    \begin{frame}
        \frametitle{normal title}
        abc
    \end{frame} 

{   
    \setbeamerfont{frametitle}{size=\small}
    \begin{frame}
        \frametitle{extra extra extraextra extra extraextra extra extra extra extra extra long title}
        abc
    \end{frame}         
}

    \begin{frame}
        \frametitle{normal title}
        abc
    \end{frame} 

\end{document}