Is it possible to scale an entire \begin{figure}?

You are looking for the macros

• \resizebox{<h-length>}{<v-length>}{<content>} and
• \scalebox{<h-scale>}[<v-scale>]{<content>}

from the graphics/graphicx packages (→ graphics manual, 3.3 “Scaling”, p. 3).

The \scalebox macro expects ratios like those you’d use in \includegraphics, you you would be using

\begin{figure}
    \scalebox{.5}{\input{plot.tex}}
\end{figure}

or, if you rather want to resize the content to a fixed width (or height),

\begin{figure}
    \resizebox{.9\linewidth}{!}{\input{plot.tex}}
\end{figure}

where ! means that the content gets resized so that it keeps its aspect ratio.

There exist also a starred version of \resizebox and you can use the lengths \height, \width, \totalheight and \depth to refer to the original sizes of the content; meaning the factor .5 could be used with \resizebox, too:

\begin{figure}
    \resizebox{.5\totalheight}{!}{\input{plot.tex}}
\end{figure}

If you're using subfigures that are scaled and want non-subfigure plots to match the others' layout, you can use the subfigure code as well with just one plot:

\begin{figure}[t]
  \centering
  \begin{subfigure}[b]{0.95\textwidth}
     \include{plot.tex}
  \end{subfigure}
  \caption{
    My caption...
  }
  \label{fig:myPlot}
\end{figure}

(In my case, using the other suggested answer left some components of the plot not matching -- maybe axis labels or legend, I forget -- thus my hacky approach.)