DWF is electronic paper, nothing more: since you cannot get a paper that
size, you are out of luck.
PLOT to a scale to a sheet size and explore what you can and cannot do with
dwf underlays inside autocad.
Or XREF this dwg file into another, since all you want is an underlay.
--
Dean Saadallah
http://LTisACAD.blogspot.com--