If you want to use font sizes in beamer smaller than 8pt, you won’t in principle be able to do so, since beamer considers them too small. For instance, options like: 

\documentclass[8pt]{beamer}

or declarations like

\setbeamerfont{normal text}{size=\small}

won’t work.

For instance, here is a minimal working example of a 5pt frame in beamer:

% Small fonts in beamer
\documentclass{beamer}

% use a vectorial font like latin modern
\usepackage{lmodern}

% set the font size to 5pt
\usepackage{scrextend}
\changefontsizes{5pt}

% load some package to show text
\usepackage{lipsum}

\begin{document}

\begin{frame}
\frametitle{Dummy but small frame}
% show arbitrary text
    \lipsum
\end{frame}

\end{document}

Compiling this code yields:

too small font in beamer