Custom Beamer Theme: Set normal font size

Question

As a follow up to an earlier question I asked, I need to be able to set the normal font size to 20pt for a Beamer presentation. The catch is, I can't use

\documentclass[20pt]{beamer}

because I want to set this font size inside a custom theme that the end users can just load without needing to supply the extra [20pt].

I already tried

\setbeamerfont{normal text}{size=20pt}

but no joy!

Answer 1

Based on the code from previous question:

\documentclass{beamer}
\usetheme{Madrid}

\defbeamertemplate*{footline}{my footline}
{
    \ifnum \insertpagenumber=1
      \leavevmode%
      \hbox{%
      \begin{beamercolorbox}[wd=\paperwidth,ht=2.25ex,dp=1ex,center]{}%
        % empty environment to raise height
      \end{beamercolorbox}}%
      \vskip0pt%
    \else
      \leavevmode%
      \hbox{%
      \begin{beamercolorbox}[wd=\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
        \usebeamerfont{title in head/foot}\insertshorttitle
      \end{beamercolorbox}}%
      \vskip0pt%
    \fi
}

\setbeamertemplate{footline}[my footline]

\AtBeginDocument{\fontsize{20}{24}\selectfont}

\title{Some Title}

\begin{document}

\begin{frame}
\titlepage
\end{frame}

\begin{frame}{Frame 1}
This is a normal frame.
\end{frame}
\end{document}