vim linebreaks in Mutt

[Rob Owens <rowens@ptd.net>]

What's the proper setting to get good line breaks in Mutt (using vim),
without manually hitting the enter key?  (So that this paragraph, for
instance, is not one long line of text, but rather several shorter lines of
text).

I currently have this in .muttrc:

set editor="vim -c 'set wrapmargin=5'"

This breaks my lines 5 characters from the right of my terminal.  Problem
is, that varies depending on how large my xterm is at the time I'm
composing the email.  I often toggle to fullscreen, and that makes my lines
get much longer.

Sorry if this is off-topic, but I've noticed there are at least a couple
Mutt users on the list.

-Rob