The Collaborative International Dictionary of English v.0.48:

Capaciousness \Ca*pa"cious*ness\, n.
   The quality of being capacious, as of a vessel, a reservoir a
   bay, the mind, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

capaciousness
    n 1: intellectual breadth; "the very capaciousness of the idea
         meant that agreement on fundamentals was unnecessary"; "his
         unselfishness gave him great intellectual roominess" [syn:
         capaciousness, roominess]
    2: spatial largeness and extensiveness (especially inside a
       building); "the capaciousness of Santa's bag astounded the
       child"; "roominess in this size car is always a compromise";
       "his new office lacked the spaciousness that he had become
       accustomed to" [syn: capaciousness, roominess,
       spaciousness, commodiousness]