ViewportSensor node returns size of 3D window in pixels.

EXTERNPROTO ViewportSensor [ 

  eventOut  SFVec2f     size_changed
]
[
  "urn:inet:parallelgraphics.com:cortona:ViewportSensor"
  "http://www.parallelgraphics.com/vrml/proto/Cortona/extensions.wrl#ViewportSensor"
]

The size_changed event is generated if the size of 3D window is changed.