Source : Webster's Revised Unabridged Dictionary (1913)

Tallness \Tall"ness\, n.
   The quality or state of being tall; height of stature.

Source : WordNet®

tallness
     n 1: the vertical dimension of extension; distance from the base
          of something to the top [syn: {height}]
     2: the property of being taller than average stature [ant: {shortness}]