Smart detection of wrap width?

Yes, I found the width option in the help pages, but I was wondering
if there was automatic setting of the wrapping according to the
current window width.

Your function works exactly as I wished.  I'll probably get smarter
with time (I hope) but would it be reasonably good practice to stick
this into ~/.Rprofile?  I don't suppose there is a way to have it
automatically invoked when the window size/positition changes?
(It's still priceless even without automatic triggering).

For ESS see
https://github.com/gaborcsardi/dot-emacs/blob/master/.emacs

Possibly, though it would take a little building.  If you were to
launch R directly when you start the xterm (loosely xterm R rather
than the default) then R would receive a SIGWINCH signal whenever
the xterm window size changes (xterm automatically sends this to its
child process).  R doesn't directly enable handling of the signal,
but there's nothing to stop you loading a dynamic library with a
little C code that set up a handler for SIGWINCH and, when it got
one, ran the equivalent of the stty command to get the new width.
The thing I've not been able to figure out is how the C code would
ever then hand that to R asynchronously.  Anyone?

At this point, and since we are in an X windows context, I think it
might be easier to use the window manager's features and write a
little macro or something that will send my setwid() command to the
active window, then assign it to a simple keystroke. Then:  resize
the window; hit the keystroke, and you're done. True, it's not fully
automatic, but it would be pretty quick and easy.

Either that or give ESS a try, using the bit that Ista offered. Or
maybe Rstudio?

Peter's got a good start, but I too would be stymied at the last
step; definitely beyond my skill.