Reduce section, subsection font size in beamer sidebar

Question

I have the same problem listed here:

Reduce fontsize beamer sidebar

Except the given answer there does not work. Thanks to the rules here at Stack Exchange, I am not allowed to post any comments asking the person who posted the answer there to clarify their example.

I am using the following settings in my beamer:

\usetheme[left]{Marburg} 
\setbeamerfont{section in sidebar}{size=\fontsize{2}{4}\selectfont}
\setbeamerfont{subsection in sidebar}{size=\fontsize{2}{4}\selectfont}
\setbeamerfont{section in sidebar shaded}{size=\fontsize{2}{2}\selectfont}

With the intention being to reduce the font size of the sections and subsections listed in the sidebar. However, these commands do not do anything. Is there an explanation or example of how they are supposed to be used in order to adjust the font size?

This also has no effect:

\setbeamerfont{section in sidebar}{size=\tiny}

maybe the default font size is already tiny?

edit: Minimum working example

\documentclass[8pt]{beamer}
\usetheme[left]{Marburg} %  width=
\setbeamerfont{section in sidebar}{size=\fontsize{2}{4}\selectfont}
\setbeamerfont{subsection in sidebar}{size=\fontsize{2}{4}\selectfont}
\setbeamerfont{section in sidebar shaded}{size=\fontsize{2}{2}\selectfont}
\begin{document}

\section{Section 1}
\subsection{Subsection 1}
\begin{frame}{}
test
\end{frame}

\end{document}

Answer 1

Font size

TeX is quite obstinate regarding font size. It suggests you to apply the font size that is available, and would not like to rescale its fonts.

If you do want to rescale fonts like what we did in MS word, we could use the fonts that we used in MS word. In some sense, those fonts are designed to be rescaled.

% !TEX program = XeLaTeX

\documentclass[8pt]{beamer}
\usetheme[left]{Marburg} %  width=
\setbeamerfont{section in sidebar}{size=\fontsize{2}{4}\selectfont}
\setbeamerfont{subsection in sidebar}{size=\fontsize{2}{4}\selectfont}
\setbeamerfont{section in sidebar shaded}{size=\fontsize{2}{2}\selectfont}

\usepackage{fontspec}
\setmainfont{Arial Unicode MS}\setsansfont{Arial Unicode MS}

\begin{document}

\section{Section 1}
\subsection{Subsection 1}
\begin{frame}{}
test
\end{frame}

\end{document}

Showing only current section's subsections

There is a _^BEAMER option called hideothersubsections that hids other section's subsection.

\documentclass[8pt,hideothersubsections]{beamer}
\usetheme[left]{Marburg}

\begin{document}

\section{Section 1}
\subsection{Subsection 1-1}
    \begin{frame}
        test \\[6cm]
        test
    \end{frame}

\subsection{Subsection 1-2}
    \frame{}
\subsection{Subsection 1-3}
    \frame{}

\section{Section 2}
\subsection{Subsection 2-1}
    \frame{}
\subsection{Subsection 2-2}
    \frame{}
\subsection{Subsection 2-3}
    \frame{}
\section{Section 3}
\subsection{Subsection 3-1}
    \frame{}
\subsection{Subsection 3-2}
    \frame{}
\subsection{Subsection 3-3}
    \frame{}

\end{document}

Answer 2

Instead of the desired solution, I found these alternatives, which will probably work as a substitute.

First, using code I found here to insert slides into the document that give a Table of Contents for just the current section and its subsections

\documentclass{beamer}
\usetheme{Marburg}

...

\AtBeginSection[]
{
  \begin{frame}
    \frametitle{Table of Contents}
    \tableofcontents[currentsection,currentsubsection]
  \end{frame}
}

\begin{document}
...

The second alternative, which I like even better, is to have the sidebar only show the subsections for the section I am currently on. This is accomplished using the following code that I found here;

\usetheme[left,hideothersubsections]{Marburg}

In this case, some of the subsections still run off the bottom of the sidebar, but it works well enough that I might leave it unless anyone else has a solution that can reduce the font size further.