According to Official Guinness Records,
The largest baseball measured 3.6 m (12 ft) in diameter, and went on display at the John Hancock All-Star FanFest, held at the David L. Lawrence Convention Center, Pittsburgh, Pennsylvania, USA, from 7-11 July 2006. The baseball is signed by famous players, such as Ted Williams and Hank Aaron (both USA).
For a complete list of 2006 records, please visit 2006 Guinness Records in United States.