If I compile the following code, I get a slide where the normal block body text size is tiny as written in the code, while the body text size of example and alert block doesn't change.
I've also tried other combinations of \setbeamerfont
, but none of them are able to change the body text size of example and alert blocks.
\documentclass[aspectratio=169]{beamer}
\usetheme{Frankfurt}
\setbeamerfont{block title}{size=\Huge}
\setbeamerfont{block body}{size=\tiny}
\setbeamerfont{block title example}{size=\Huge}
\setbeamerfont{block body example}{size=\tiny}
\begin{document}
\begin{frame}
\begin{exampleblock}{Title}
Test
\end{exampleblock}
\begin{alertblock}{Title}
Test
\end{alertblock}
\begin{block}{Title}
Test
\begin{itemize}
\item Item
\end{itemize}
\end{block}
\end{frame}
\end{document}