The biggest Lego set appeared in 2008 as number 10189, the "Taj Mahal." It had 5,299 pieces. This is the largest set in "pieces." The largest Lego set purely in "size" is number 10179, the "Millennium Falcon," that was sold in 2007 and had 5,195 pieces.
The Taj Mahal, Millennium Falcon and the next seven largest sets are no longer in production but can still be purchased in specialty and collectors market places. As of 2014, the largest set that can still be purchased from Lego directly is set number 10224, "Town Hall," with 2,766 pieces and a suggested retail price of $199.99.