The Collaborative International Dictionary of English v.0.48:

Wideness \Wide"ness\, n.
   1. The quality or state of being wide; breadth; width; great
      extent from side to side; as, the wideness of a room. "I
      landed in a small creek about the wideness of my canoe."
      --Swift.
      [1913 Webster]

   2. Large extent in all directions; broadness; greatness; as,
      the wideness of the sea or ocean.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wideness
    n 1: the property of being wide; having great width [syn:
         wideness, broadness] [ant: narrowness]
    2: unusual largeness in size or extent or number [syn:
       enormousness, grandness, greatness, immenseness,
       immensity, sizeableness, vastness, wideness]