Here is a thread from 2005 on the same topic. I believe the biggest in terms of land area was RUH
at 225 sq km. DMM
was also discussed to be 776 sq km, but according to Reply #22 it only occupies 43 sq km of the 776 sq km. Even when you specify largest in terms of land area, one can still argue over definitions and criteria such as land that the airport owns or land that it actually uses...
[Edited 2012-01-06 12:44:32]
Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,737,738,739;741,742,743,744,747SP;752,753;762,763;772,773.