User blog comment:Callofduty4/Ghosts PC Title Update 11/19/13/@comment-122.150.176.236-20131121075044/@comment-91.154.30.173-20131121194650

That article is from October 25, this CharlieIntel post has an image that says it's 21,1 (MP) + 7,5 (SP) gigabytes after the game came available to preload: http://www.charlieintel.com/2013/11/01/pre-load-call-of-duty-ghosts-on-steam-now/

Oddly though the requirements on the store page in Steam haven't been updated.