OuterFile.tex
--------------
\documentclass{article}
\usepackage{mwe}
\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}
\usepackage[]{subfig}

\begin{document}

\includegraphics[width=3cm]{example-image-a}

\end{document}