The Collaborative International Dictionary of English v.0.48:

Enormousness \E*nor"mous*ness\, n.
   The state of being enormous.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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