\starttext

\definepapersize[main][A4][A4]
\definepapersize[diagram][A4,landscape][A4,landscape]

Some stuff in portrait.

\page 
\setuppapersize[diagram]
\placefigure
    [][figure:SomeFigure]
    {Figure's caption.}
    {\externalfigure[Figure.svg][width=\textwidth,height=\textheight,factor=max]}
\page
\setuppapersize[main]

More stuff in portrait.

\stoptext