a lot
A regulation-size tennis court is a rectangle, 78 feet (23.77 m) long and, for singles matches, 27 feet (8.23 m) wide. For doubles matches, the court is 36 feet (10.97 m) wide.
There is no 'average' size for a tennis court, the regulation size is 36' x 78'
the average size varies particularly to about 11 by11 metres