Airplanes are the biggest things in any museum anywhere. That’s why the National Air and Space Museum, in Washington, D.C., as big as it is, isn’t nearly big enough. On December 15, it will multiply its space by opening a new museum annex, the Udvar-Hazy Center, at Dulles Airport in northern Virginia. Udvar-Hazy (named after its principal donor) will be 984 feet long and 10 storieshigh and will eventually hold more than 300 planes and spacecraft.