https://www.gamespot.com/articles/black-ops-4-pre-load-is-live-on-ps4-xbox-one-and-p/1100-6462452/

Ahead of its launch on October 12, Activision's Call of Duty: Black Ops 4 is now available to pre-load across Xbox One, PlayStation 4, and PC if you pre-order the game. GameSpot discovered that it weighs in at 49.33 GB on Xbox One, and the file size is presumably similar on PS4 and PC.

In fact, Activision said in a post on Reddit that the file size for the digital full game is 55 GB, but this appears to be an average across PS4, Xbox One, and PC. If you're picking up Black Ops 4 on a physical disc, you cannot pre-load, of course. Additionally, the physical disc version requires a hefty 50 GB day one update, so that's something you may want to consider.