Another interpretation of your question is "how do I print a picture of the
cell that shows how diameter changes along each section."
Many of NEURON's GUI tools include a Shape plot that displays a
projection of the cell onto the xy plane. By default, Shape plots show
just the centroid of each section. This saves time, but no matter how
much you zoom in, or how large you expand the Shape plot, it's just
a bunch of skinny lines.
To see diameter variation, click on the Shape plot's menu box (in the
upper left corner) and select Shape style / show diam. Branches that
are rendered smaller than a pixel wide may completely disappear from
your computer monitor, but if you zoom in or print to a PostScript
printer, you'll see that they still exist.
If you want to save the Shape plot to a graphics file, the best format
is either Postscript or encapsulated Postscript. You can do this with
the Print & File Window Manager, but that's a different story (go to
NEURON's FAQ list
and find the link
How do I print a hard copy of a NEURON window?
Many vector graphics ("draw") programs, such as CorelDraw, can
import the Postscript and idraw (encapsulated Postscript) files that
NEURON generates. You can also print graphs of voltage, current, etc.
to Postscript or idraw files, for subsequent revision and annotation with
a vector graphics program.