The Collaborative International Dictionary of English v.0.48:

Commodiousness \Com*mo"di*ous*ness\, n.
   State of being commodious; suitableness for its purpose;
   convenience; roominess.
   [1913 Webster]

         Of cities, the greatness and riches increase according
         to the commodiousness of their situation. --Sir W.
                                                  Temple.
   [1913 Webster]

         The commodiousness of the harbor.        --Johnson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

commodiousness
    n 1: 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]