External.tex
---------
\documentclass{article}
\usepackage{standalone}
\begin{document}
\def\FigSize{.23}
\begin{figure}[htbp!]
\centering
\input{Internal.tex}
\end{figure}
\end{document}

Internal.tex
-----------
\documentclass[preview]{standalone}
\usepackage{mwe}
\begin{document}
\includegraphics[width=3cm]{example-image-a}
\end{document}