I'm afraid my advice on large-format Postscript printing
might be considered "vapour-ware" . All I can relate is
that the following tools are available to create a
postscript grass output:
1. GRASS4.x paint ppm driver, which creates a paint.ppm
file in users home directory.
2. The pbmplus program ppmtops (or pnmtops) will convert
the paint.ppm file into a color postscript file which
should be suitable for any color postscript printer.
3. If larger scale is desired one might use the pbmplus
program ppmscale (or pnmscale) to increase the size of
the paint.ppm file prior to conversion to postscript,
but I'm not sure what effect that will have on the
appearance of the output. An alternative would be some
notes I saw earlier in the week relating to correcting
the way the paint ppm driver treats scale and legend.
Unfortunately I neglected to save them 
.
Mark Johnson - USACERL