In what seems to be the largest device monitored (and controlled?) by Linux is the CERN's Large Hadron Collider, as displayed in this BBC coverage of the experiment. This just proves how Linux can be used in mission-critical systems, debunking claims by others that it is not ready for serious computing. (Incidentally, the LHC is also controlled/managed by applications written on Java!)
Can you name another device larger than the LHC that is controlled, monitored or managed by Linux-based computers?
[Image copyrighted and from.ch]