It’s 3.2GB
Last night, after redeeming my code for Genesect, I went through and got my Pokemon Bank and my memory card ready for Sun & Moon. Wait, memory card? Yep, even though I’m rocking a 32GB, either version of the game will take up 3.2GB, or 26,214 blocks in total, and that’s not including the extra space reserved for the demo — with the caveat that you need both installed to transfer over the demo items.
But if you’re all clear already, you can go ahead and preload either version now, provided that you pre-ordered on the eShop. You’ll get 12 Quick Balls for your trouble, which is pretty lame, but something, I guess. Honestly I’d rather just have the game loaded up and ready to go for release rather than run into some snag like a three hour download.
If you aren’t all clear, do some planning and consider picking up a bigger memory card. Up to 32GB is “supported officially by Nintendo,” but you can go higher if you want.
November 2016 [Serebii]