Above are schematics of the colossal Mothership from the movie ID4. Each pixel in the image equals about 4 square kilometers (2km to a side). I have portrayed the Mothership as being roughly 800km long. Although there do not seem to be any reliable sources for this length, I was able to come to this conclusion in two ways:

1.) During the movie, it is stated that the ship is approximately 550km in diameter. From observations of photgraphs of the ship, we know it to be oblong in shape and therefor this measurement must be of one axis or the other.

2.) Upon observation of numerous images of the underside of the mothership we can infer from the known size of the city destroyers (24km dia.) that the longer axis would probably come to the length of roughly 800km, while the shorter axis does indeed come to approximately 550km. This keeps in line with the dimensions stated in the movie.

The movie also goes on to say that the ship has a mass of roughly one 4th the size of the moon, which seemed unlikely to me until my wife suggested a hull created from some incredibly dense material, which makes sense and would protect the vessel from interstellar debris and radiation.

The ship is at least partially hollow, much of the inside comprising of a vast cavern, a gargantuan open area tens of thousands of cubic kilometers in volume that seems to maintain a misty internal atmosphere. Gigantic towers of unstated purpose seem to span the entire height of the chamber at random intervals.