According to Official Guinness Records,
The largest high-definition video screen is an LED display which measures 250 m x 30 m (820 ft x 98 ft) for a total surface area of 7,500 m² (80,729 ft²). It is located at The Place, Beijing, China. The screen is actually composed of five (still extremely large) screens that are arranged in a line to produce one continuous image.
For a complete list of 2009 records, please visit 2009 Guinness Records in China.