Beamer: Have text box automatically shrink text size in sty file

Question

I am in the process of creating a custom beamer theme, and in the beamerinnertheme.sty file I am trying to define the title, author, and date text boxes to automatically shrink their text to fit a pre-determined box size. From what I've found it looks like I'd use the tcolorbox package. I've defined the section where I want my box with

\newtcboxfit{\mybox}{width=\linewidth,colback=white,colframe=white}

% Title page
\defbeamertemplate*{title page}{texsx}[1][]
{ 
   \vskip2.5cm%
    \begin{beamercolorbox}[wd=12cm,leftskip=1cm,sep=8pt,#1]{title goes here}
      \usebeamerfont{\mybox{title}\inserttitle}\par%
    \end{beamercolorbox}%
    \vskip0.75cm%
    \begin{beamercolorbox}[wd=12cm,leftskip=3cm,#1]{author}
      \usebeamerfont{author}\insertauthor%
    \end{beamercolorbox}
     \vskip0.2cm%
    \begin{beamercolorbox}[wd=12cm,leftskip=3cm,#1]{date}
      \usebeamerfont{author}\insertdate%
    \end{beamercolorbox}
  \vfill
}

But this does not produce what I'm looking for, mostly being I can't make it so that the box defined takes the title input as an argument.

Answer 1

A couple of syntax problems:

• The mandatory argument of the beamercolorbox environment has to be a beamer colour. Unless you have a custom defined colour names title goes here, I suspect you'd rather want something like \begin{beamercolorbox}[...]{title}

• The \beamerfont macro needs as argument a beamer font, not your box, so it should be \usebeamerfont{title}

• The argument of your box should be the text to be placed in the box, in your case this text is stored in \inserttitle, so you want to use \mybox{\inserttitle}

• The fitting library of tcoolorbox needs to be given a height of the box. If you don't specify a height, it will assume a square box - given the aspect ratio of beamer frames you'd never see a shrinkage in this case.

---

\documentclass{beamer}

\usepackage[most]{tcolorbox}

\newtcboxfit{\mybox}{colback=red, colframe=red,width=\linewidth, height=1cm}

% Title page
\setbeamertemplate{title page}{ 
   \vskip2.5cm%
    \begin{beamercolorbox}[wd=\linewidth,leftskip=0cm,sep=8pt]{title}
      \usebeamerfont{title}\mybox{\inserttitle}\par%
    \end{beamercolorbox}%
    \vskip0.75cm%
    \begin{beamercolorbox}[wd=\linewidth,leftskip=3cm]{author}
      \usebeamerfont{author}\insertauthor%
    \end{beamercolorbox}
     \vskip0.2cm%
    \begin{beamercolorbox}[wd=\linewidth,leftskip=3cm]{date}
      \usebeamerfont{author}\insertdate%
    \end{beamercolorbox}
  \vfill
}

\title{text very long text very long text text very long text very long text text very long text very long text}
\author{names}


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

(to prevent your theme from exploding, you might want to check if the fields you insert are actually filled or if they are empty...)

If you use a font which scales easily, e.g. which does not have different optical sizes, it might be easier to use the adjustbox package:

\documentclass{beamer}

\setbeamerfont{title}{family=\fontfamily{LibertinusSans-LF}\selectfont}% just the first font I could think of which is missing optical sizes, not a recommendation to use this font

\usepackage{adjustbox}
\setbeamercolor{title}{bg=red}

% Title page
\setbeamertemplate{title page}{ 
   \vskip2.5cm%
    \begin{beamercolorbox}[wd=\textwidth,leftskip=0cm,sep=8pt]{title}
      \usebeamerfont{title}\adjustbox{max width=\linewidth-16pt}{\inserttitle}\par%
    \end{beamercolorbox}%
    \vskip0.75cm%
    \begin{beamercolorbox}[wd=\linewidth,leftskip=3cm]{author}
      \usebeamerfont{author}\insertauthor%
    \end{beamercolorbox}
     \vskip0.2cm%
    \begin{beamercolorbox}[wd=\linewidth,leftskip=3cm]{date}
      \usebeamerfont{author}\insertdate%
    \end{beamercolorbox}
  \vfill
}

\title{text very long text very long text text very long text very long text text very long text very long text}
\author{names}


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