The Jargon File (version 4.4.7, 29 Dec 2003):

Big Room
 n.

    (Also Big Blue Room) The extremely large room with the blue ceiling and
    intensely bright light (during the day) or black ceiling with lots of tiny
    night-lights (during the night) found outside all computer installations. ?
    He can't come to the phone right now, he's somewhere out in the Big Room.?


The Free On-line Dictionary of Computing (30 December 2018):

Big Room

    The extremely large room with the blue
   ceiling and intensely bright light (during the day) or black
   ceiling with lots of tiny night-lights (during the night)
   found outside all computer installations.  "He can't come to
   the phone right now, he's somewhere out in the Big Room."

   (1996-03-04)