Very big.
How big though? Honestly, I'm not too sure.
As popular as PUBG has been, it's still a game that imo, has yet to truly prove itself as a zeitgeist game that has broken into mainstream awareness. And yes, I say that fully aware that it'll comfortably sell >10 mil copies on Steam alone by the end of the year.
In the same vein that games like H1Z1 and Ark are silent titans that has built up an audience >7 million on Steam alone without creeping into mainstream awareness, PUBG is the game with the biggest potential of these silent titans to break out.
I think, that by default, by the end of this year, it should outdo Ark's performance on XB1 by a factor of three, easily. 3+ million on XB1, roughly 10-15% attach rate at launch.
Whether or not it compounds beyond that, depends very much on how much MS will be supporting its push into broader gaming awareness. Also, will it get a retail release?