ID4 City Detroyer - 24km in diameter
Above are schematics of one of the massive ID4 City Destroyers. These invasion ships were stated in the movie as being roughly 15 miles (24km) in diameter each.

In the center on the underside of the ship is the schism. It is this aperture from which the 'Hammer' was used in the movie to destroy most of the largest cities on the planet. The 'Hammer' (as it was apparently known to those on the set) is portrayed as an energy beam that forms a destructive shockwave upon surface impact that seems to be used in terraforming and offensive maneuvers.

Each one of these ships likely carries tens of thousands of individuals and small fighter craft.