If we go for a higher number and want to enforce it (and I'm not
saying we should), we should try to add a suitable setting to the C
source files (in the local variables section), or maybe provide a
.dir-locals.el file, for the benefit of Emacs users. Of course, not
everyone uses Emacs.
I don't think we should enforce any number, but rather try to provide
a guideline. After Mark's email, I'd be happy to put the guideline at
70 chars, and see how it goes. I realize that, in all fairness, it isn't
the most important issue to be discussing ;-).