The option seems to be active for those that pre-ordered digitally on the PSN, and only in some regions so far. Reports mention that if they have automatic downloads active, the PS4 queues the beta all by itself. Of course the servers aren’t open yet, so the game is not playable.
The initial download is 179 Mb, followed by a second download of 13.2 Gb. Once unpacked and installed it takes up 14.3 Gb on the hard disk. By comparison the download for Xbox One is a little smaller, weighing 12.63 Gb.
If you pre-ordered the game digitally, you may want to check if your pre-load is already available. If it’s not, it could appear soon.