GNUS was written for B news, which would generate the `Lines:' header. C news doesn't. There is a comment in C news's `inews' that you can uncomment to enable this functionality. Or you can have GNUS generate the header, for example:
;; idea by firstname.lastname@example.org (Jack Bryans) (defun add-lines-header () ;; Count the number of lines in the current posting and insert the ;; header line Lines into the message. (save-excursion (goto-char (point-min)) (if (search-forward "\n\n") ;; ***** I suspect this is wrong ***** (let ((lines (count-lines (point) (point-max)))) (forward-line -1) (insert-string "Lines: " lines "\n")))))
Mike Williams <email@example.com> has written something similar.
Ronald Florence <firstname.lastname@example.org> has a patch for GNUS that makes it calculate the `Lines:' header for incoming articles when necessary that works for sites with local news spools.
David Lawrence <email@example.com> says that GNUS 3.14.1 generates Lines if gnus-news-system is Cnews.