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}
figureenvironment? – jacobq Oct 07 '13 at 18:55\scaleboxalso works with .pstex_t files exported from xfig (the .pstex_t file includes --with\includegraphics-- a .pstex file).\resizeboxdoes not work. – kavadias May 29 '16 at 01:11