You can already pre-install The Witcher 3: Wild Hunt to your console, even though you won't be able to play it for a month. But just how much space would you actually need for the game? On Xbox One you need at least 25GB free space to download the game, though users say that the final installation takes a bit over 30GB in total.
On PS4 you need more space, as the US Playstation Store lists the download size to be 35GB. The European Store still lists the size as 50GB, but it's look likely that this isn't accurate.
It's also worth noting that the Xbox One version of the game, which as we mentioned before can be pre-ordered and downloaded a full month before launch, is available at a 10% discount (so in the UK it's currently £40.49 instead of £44.99).