The Collaborative International Dictionary of English v.0.48:

Tallness \Tall"ness\, n.
   The quality or state of being tall; height of stature.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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