Even at its smallest, this new ship is nearly the length of the space station Deep Space 9 (1452 meters), and falls a little short of the Argus Array (2400 meters).
In my mind, the Enterprise is 300m long. The new ship is no bigger than the Excelsior. That is all.
On another note, one could probably gauge the size of the dreadnought by measuring the size of the deflector dish compared to the height of Alcatraz. But if the above estimate is accurate, then it would basically fill the bay between Alcatraz and the mainland.