The Collaborative International Dictionary of English v.0.48:

Vastness \Vast"ness\, n.
   The quality or state of being vast.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vastness
    n 1: unusual largeness in size or extent or number [syn:
         enormousness, grandness, greatness, immenseness,
         immensity, sizeableness, vastness, wideness]