Fallout 4 is going to be a huge, huge game. The number Bethesda are quoting at the moment is 400 hours 'complete the game'. We can assume that number is to see all the content, rather than the story mode Still, I don't doubt that you will be able to be lost for ages in the Wasteland.
That is why I am quite surprised to learn that the file size for the whole game on Xbox One is only going to be 28.12GB. This was spotted on the Microsoft store (via GameSpot), and you would have to imagine there would be some kind of parity between PS4 and PC too.
For reference, Titanfall had around a 55 GB install, which is kind of mind blowing when you consider that that game was just a handful of multiplayer maps. So yeah… interesting.