How to affect plain Beamer last page with geometry margin=0pt?

Question

I am trying to setup the margin zero of my form Beamer last page. Code

\documentclass{beamer}
\usepackage[english]{babel}
\usetheme{Berkeley}

% TODO how to affect the last page
\usepackage[margin=0pt]{geometry}% http://ctan.org/pkg/geometry

\usepackage{tabularx}
\usepackage{adjustbox}
\newcommand*\rot[1]{\rotatebox{90}{#1}} % https://tex.stackexchange.com/a/201117/13173

\begin{document}

% https://tex.stackexchange.com/a/385265/13173
\pdfpagewidth 21cm
\pdfpageheight 29.7cm

% Show summary here from Page 1 and 2
\begingroup
\setbeamertemplate{navigation symbols}{}%remove navigation symbols
\section{Summary}
\begin{frame}[plain]
\vskip1cm
\begin{minipage}{19cm}
    \frametitle{START}
    \begin{adjustbox}{max width=\textwidth}
    \begin{table}[hpt]
        \setlength\tabcolsep{.1pt} % default value: 6pt % https://tex.stackexchange.com/a/201117/13173
        \begin{tabular}{|p{.5cm}|l|l|l|l|l|l|l|p{.5cm}|}
        \hline
        \rot{{\tiny\textbf{Avain}}} 
        & \textbf{V2} & \textbf{V1} & \textbf{V0} 
            & \textbf{START} 
            & \textbf{P1} & \textbf{P2} & \textbf{P3} 
            & \rot{{\tiny\textbf{Kriittinen}}} \\ \hline
         &  &  &  & 1. taidot &  &  & & \\ \hline
        \end{tabular}
    \end{table}
    \end{adjustbox}

\end{minipage}
\end{frame}

% these below eventually are not needed, if this is the last slide of your document
\makeatletter
\pdfpagewidth \beamer@paperwidth
\pdfpageheight \beamer@paperheight
\makeatother
\endgroup

\end{document}

Output in Fig. 1

Fig. 1 Output

Doing \setlength{\parindent}{0pt} does not help.

OS: Debian 9
TeXLive: 2017

Answer 1

beamer already loads the geometry package, so you cannot load it again with different settings - the error message caused by this is quite explicit about the option clash.

Quick hack:

\documentclass{beamer}
\usepackage[english]{babel}
\usetheme{Berkeley}
\usepackage{lipsum}

\begin{document}

\pdfpagewidth 21cm
\pdfpageheight 29.7cm

\setbeamertemplate{navigation symbols}{}

\begin{frame}[plain]
    \vskip3cm

    \hspace*{-2cm}%
    \begin{minipage}{20.75cm}
       \lipsum
    \end{minipage}
\end{frame}

\end{document}

(I replaced your table with random text, as this questions is about the margin and not about debugging your adjustbox usage. BTW floating options don't make any sense in beamer)