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}
