Not very tall. The floorspace is not significantly different from the Model S, since its built on the same platform. The MS is tall enough with the seat folded, but the space would only be in the 4 foot range with the seat still up.
But that is making the assumption that the seat can't be configured into a flat cargo space, which I believe it can. The 2nd row seat, IMHO, will slide all the way back, then tilt forward such that the back of the seat is flush with the back of the 3rd row seat back when it is folded down. I think the back of the 2nd row seat is carpeted, and the whole thing tilts, seat and all, to a forward flat surface.
I think the length of the space will be the same as the MS, but the width will be greater since the whole floor will be flush with the tops of the wheel wells.