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 jbryans@beach.csulb.edu (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 <mike-w@cs.aukuni.ac.nz> has written something similar.
Ronald Florence <ron@mlfarm.com> 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 <tale@uunet.uu.net> says that GNUS 3.14.1 generates Lines if gnus-news-system is Cnews.