I looked into pgfcircbipoles.tex and found values like \ctikzvalof{bipoles/capacitor/height}, but this seems to be factors (?).
How do I draw a line as long and as wide as a capacitor line?
\documentclass[margin=5pt]{standalone}
\usepackage{circuitikz}
\begin{document}
%\def\t{\ctikzvalof{sources/symbol/thickness}}% test
%\def\t{\ctikzvalof{bipoles/cuteswitch/thickness}}% test
\def\t{\ctikzvalof{bipoles/thickness}}% test
\def\h{\ctikzvalof{bipoles/capacitor/height}}% test
\begin{circuitikz}
\draw (0,0) to[capacitor, name=A] (2,0) to[R] (4,0);
\draw[red, line width=\t, opacity=0.5] (A.north east) node[above, inner sep=0.5pt]{\t=t}
-- +(0,-\h) node[near start, right]{\h=h};
\end{circuitikz}
\end{document}


