Owlcat Games announced it's open season and you can start putting Pathfinder: Wrath of the Righteous on your machine to be ready for the launch day.
Pathfinder: Wrath of the Righteous is a hotly anticipated title due to the success of Pathfinder: Kingmaker, meaning you will probably want to get your hands on it as soon as possible. Provided you're not a backer that already has access to the game, your time will come on September 2, 2021. Before that, however, you are able to download the game in its entirety, in order to skip waiting for the download to finish.
That said, keep in mind that Steam will need to unpack the game so some waiting will be inevitable on the launch day. If your download speed is fast enough, you can just download Wrath of the Righteous on the launch day as it's likely to be faster than unpacking, which can take hours on end. On top of that, Day One patch will become available at the same moment the game gets unlocked so the actual preloading value is rather dubious here.
Speaking of which, Wrath of the Righteous will unlock on September 2, 2021, at 9:00 AM PDT / 12:00 PM EDT (noon) / 5:00 PM BST / 6:00 PM CEST.
As for the download size itself, Steam mentions you need about 50 GB of free space but the actual download will probably be smaller as games often suggest having more space to avoid failing updates if a disk is at full capacity.