Document

\begin{filecontents*}{pic\jobname.tex}
    \documentclass{standalone}
    \usetikzlibrary{shapes}
    \begin{document}
          \color{red}
            inside
        \begin{tikzpicture}[x=1em, y=1em]
            \draw (0,0) rectangle node{Test} (2,2);
        \end{tikzpicture}
    \end{document}
\end{filecontents*}

\documentclass{article}
\usepackage{tikz}
\usepackage{xcolor}
\usepackage[fontsize=14pt]{scrextend}
\usepackage[mode=build, subpreambles=false]{standalone}
\begin{document}
\section*{d}
    fontsize = 14pt \includestandalone{pic\jobname}

    \huge fontsize huge \includestandalone{pic\jobname}

    \tiny fontsize tiny \includestandalone{pic\jobname}
\end{document}