The Collaborative International Dictionary of English v.0.48:

Extensiveness \Ex*ten"sive*ness\, n.
   The state of being extensive; wideness; largeness; extent;
   diffusiveness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extensiveness
    n 1: large or extensive in breadth or importance or
         comprehensiveness; "the might have repercussions of
         unimaginable largeness"; "the very extensiveness of his
         power was a temptation to abuse it" [syn: largeness,
         extensiveness]