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Webster 1913 Edition

Noun.

The quality of being capacious, as of a vessel, a reservoir a bay, the mind, etc.

Noun.

Wideness; largeness; as of a vessel.

Extensiveness; largeness; as of a bay.

Comprehensiveness; power of taking a wide survey; applied to the mind.

capaciousness ‎(usually uncountable, plural capaciousnesses)

The quality of being capacious.
- 1818 George Crabb - English Synonyms Explained, in Alphabetical Order
  Capacity is the abstract of capax, receiving or apt to hold ; it is therefore applied to the contents of hollow bodies : capaciousness is the abstract of capacious and is therefore applied to the plane surface comprehended within a given space. hence we speak of the capacity of a vessel; and the capaciousness of a room.