This is the new BT Smart Hub X which I received yesterday for the BT Ultrafast Broadband trial. The box should fit through most letter boxes, but the postman did knock on the door to deliver mine.
Inside the box, showing a surprisingly large broadband hub. I placed a £2 coin on the hub as a size reference:
And here, my current Smart Hub 4 placed side-by-side with the new Smart Hub X: