i just got this randomly off the internet...
'If by "largest" you mean "most square footage," then the trophy goes to Madrid's Royal Palace, which boasts 2800 rooms in 135,000 square meters (or over 1,450,000 square feet). Taking some 28 years to construct, the palace has 44 staircases, 870 windows, and 270 balconies. Although it is the official residence of the King of Spain, the royal family no longer resides there and the palace is now used only for ceremonial occasions.
While not a traditional palace (it isn't a royal residence), the Palace of Parliament in Bucharest, Romania, is the biggest building in Europe. In fact, it's the second-largest building in the world -- only the U.S.'s Pentagon is bigger in area. The Palace of Parliament dwarfs other contenders for the title at a whopping 330,000 square meters. Begun in 1984, this edifice was created by the Communist dictator Nicolae Ceausescu, and was renamed the "Palatul Parlamentului" after his regime was ousted'
James