how to make procmail generate content-length?

we use procmail for local delivery, and would like to get it to generate
the content-length header, if one doesn't exist.  mailtool at least gets
confused and merges messages together if there is no message body.  i
can't seem to find a way to make that happen, nor can i seem to figure
out a way to make a procmailrc rule via formail to do it, either.

any clues from out there?