The clockfaces of the Big Ben have a diameter of 7.5m.
Four
The faces/dials are 7m (23') in diameter. The numerals are 0.6m (2') high, and the minute hand is 4.25m (14') long. It is the biggest in the World.
The actual clock face does not seem to have a name. The Tower, clock and Bells are all traditionally called Big Ben.
Strictly speaking, Big Ben is the name of the bell that is contained within the clock tower of the Houses of Parliament. It is a common misnomer to call the clock Big Ben.