The distance to the Orion Nebula and its associated molecular clouds
  is roughly 450 pc.


  Q:  Using the angular size of "cloud A" in the right panel above,
      calculate the linear size of that GMC.  Is the term "Giant"
      a good description?



         To a decent approximation, the linear size is given by


               L   =   d * tan(theta)

                   =   (420 pc) * tan(7 degrees)

                   =   51 pc


         That's ... pretty big.  "Giant Molecular Cloud" is an accurate
         description!