Code:
\documentclass[12pt]{article}
\usepackage{float}
\usepackage[american,siunitx]{circuitikz}
\begin{document}
\noindent
Hello\newline
\footnotesize
This is small.
\begin{figure}[H]
\centering
\begin{circuitikz}
\draw(0, 0) to [V, v_= $v_g\left(t\right)$] ++(0, -3) ;
\draw(0, 0) to [R, l^= $R$] ++(3,0) to [C, l^=$C$] ++(0, -3) -- ++(-3,0);
\end{circuitikz}
\caption{A simple low-pass filter.}
\label{RCFilter}
\end{figure}
\noindent
This is small.
\end{document}
I realized that when I used the \footnotesize command, the text changes, but the figure captions do not. Is there a way of getting the font of the figure's text to be changed, the same effect as if I change \documentclass[12pt]{article} to \documentclass[10pt]{article}?
Note: I am not sure if this is true, but I think that under my situation, the section headers' fonts may remain unchanged besides the text in the figures.

footnotesizecommand, I wanted the text in the figure and the caption to be of the same font as the text outside of the figure. – Superman Jun 07 '20 at 05:46