TikZ: How to pass the font size of the calling document to the called standalone tikz file?

Question

I would like to make any change in the font size of the main calling document affect the font size of the text and the drawing dimensions of the standalone file since any current change in the article font size has no effect on the standalone file.

The following images shows no change in the text font size of the standalone tikz drawing despite changing it in the calling document.

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

\documentclass{article}
\usepackage[fontsize=14pt]{scrextend}
\usepackage[mode=build, subpreambles=false]{standalone}
\begin{document}
    fontsize = 14pt
    \includestandalone{pic}
\end{document}

---

Update 1 (Compiling case (d) of @Cicada's answer)

\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}

Output

4 almost empty pages

https://docdro.id/4t5urCy

log file of main document

https://pastebin.com/ws9Fj0tA

log file of standalone file

https://pastebin.com/Dt7phZrU

Answer 1

Not an answer because the question as far as I can understand it relates to the standalone image(PDF) to be included in the main file, with the proviso that the standalone PDF be compiled only once to save compilation time.

If, instead, standalone source code is used, the standalone PDF will inherit any applicable main document settings like fontsize but the standalone PDF will be (re)compiled on each run of the main document.

---

If source code is used:

(Original answer)

Don't use font=\normalsize inside your tikz picture.

(a) Your question code does not load tikz at the correct time: "Environment tikzpicture undefined.", says the log. tikz package needs to be loaded before tikz commands can be used.

No illustration (a).

(b) \begin{tikzpicture}[x=1em, y=1em, font=\normalsize] sets the fontsize inside the rectangle, overriding the font size used in and by the calling file at that point in the processing. The rectangle size is set to relative font size (em) so expands or contracts with it.

MWE

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

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

    \huge fontsize huge \includestandalone{pic\jobname}

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

(c) Do \begin{tikzpicture}[x=1em, y=1em] so as not to override the font size.

MWE

\begin{filecontents*}{pic\jobname.tex}
    \documentclass{standalone}
    \usetikzlibrary{shapes}
    \begin{document}
        \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[fontsize=14pt]{scrextend}
\usepackage[mode=build, subpreambles=false]{standalone}
\begin{document}
\section*{c}
    fontsize = 14pt \includestandalone{pic\jobname}

    \huge fontsize huge \includestandalone{pic\jobname}

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

(d) Notice that inside the included file, formatting commands (e.g., colour) are restricted to the file itself. This implies that the contents of the included file are processed in their own group (as if with {...} or equivalents).

MWE

\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}

(e) Fontsize option for the documentclass works as expected.

MWE

\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[12pt]{article}
\usepackage{tikz}
\usepackage{xcolor}
\usepackage{scrextend}
\usepackage[mode=build, subpreambles=false]{standalone}
\begin{document}
\section*{e}
        \fontname\font\\
    fontsize = 12pt \includestandalone{pic\jobname}

    \huge       \fontname\font\\ fontsize huge \includestandalone{pic\jobname}

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