The format of the `papersize' special is
\special{papersize=width,height}
width is the horizontal size of the page, and height is the vertical size. The dimensions supported are the same as for TeX; namely, in (inches), cm (centimeters), mm (millimeters), pt (points), sp (scaled points), bp (big points, the same as the default PostScript unit), pc (picas), dd (didot points), and cc (ciceros).
For a US letter size landscape document, the papersize would be:
\special{papersize=11in,8.5in}
An alternate specification of landscape:
\special{landscape}
This is supported for backward compatibility, but it is hoped that
reventually the papersize comment will dominate.
Of course, such a \special only informs Dvips of the desired
paper size; you must also adjust \hsize and \vsize in your
TeX document typeset to those dimensions.
The papersize special must occur somewhere on the first page of
the document.