Masjid al-Haram is the largest mosque in the world. Located in the city of Mecca, it surrounds the Kaaba, the place which Muslims worldwide turn towards
while offering daily prayers and is Islam’s holiest place. The mosque is also known as the Grand Mosque. The current structure covers an area of 4,008,020 square metres (990.40 acres) including the outdoor and indoor praying spaces and can accommodate up to four million Muslim worshippers during the Hajj period, one of the largest annual gatherings of people in the world.
source : http://thewondrous.com