LaTeX - Automatically scale down an area so that it will be inside a page if it's too big?

you can do something like

\includegraphics[width=\textwidth]{figure}

or

\includegraphics[width=0.33\textwidth]{figure}
\includegraphics[width=0.33\textwidth]{figure}
\includegraphics[width=0.33\textwidth]{figure}

You may try the following macro:

\maxsizebox{〈width〉}{〈height〉}{〈content〉}

It only resizes the content if its natural size is larger than the given 〈width〉 or 〈height〉, but does not change the aspect ratio. It is part of the adjustbox package. So you need to append to the preamble of your document:

\usepackage{adjustbox}

You can further read about it here.