Tried to see if I could figure out a size, but without a known dimension in game, it's hard to do trig. On a related note, there is some parallax in the back drop. If you run from one side of the atrium of the tower to the other, you can see minute shifts in the sky box. Too small to measure with good enough precision, but indicative of the absurd amounts of attention that was put into Destiny's visuals.