Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

capaciousness
     n : spatial largeness (especially inside a building) [syn: {roominess},
          {spaciousness}, {commodiousness}]