The pituitary gland volume changes depending on hormonal status and what would be a normal gland in one demographic would be grossly abnormal in another. 
Generally speaking, young adults have larger glands than older individuals, and hormonally active individuals (puberty / pregnancy) have the largest glands. These plump glands completely fill the pituitary fossa, and have a convex upper border, whereas older individuals will have a largely empty pituitary fossa, with a deflated and thinned gland lying in the floor of the sella. 
Although one should always be wary of measurements, they can serve to quantify what may otherwise seem overly subjective impressions. These are reasonable maximal figures for the height of the gland ¹:

• children (<12 years) : 6mm (upper surface flat or slightly concave)
• puberty : 10mm (upper surface convex; more striking in females)
• young adult
  • male : 8mm
  • female : 9mm
  • pregnancy : 12mm
• older adult ( > 50 years) : gradually decreases in size.
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