The Collaborative International Dictionary of English v.0.48:

Immensity \Im*men"si*ty\, n.; pl. Immensities. [L. immensitas:
   cf. F. immensit['e].]
   The state or quality of being immense; inlimited or
   immeasurable extension; infinity; vastness in extent or bulk;
   greatness.
   [1913 Webster]

         Lost in the wilds of vast immensity.     --Blackmore.
   [1913 Webster]

         The immensity of the material system.    --I. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immensity
    n 1: unusual largeness in size or extent or number [syn:
         enormousness, grandness, greatness, immenseness,
         immensity, sizeableness, vastness, wideness]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "immensity":
   all-comprehensiveness, all-inclusiveness, ampleness, amplitude,
   boundlessness, bulk, countlessness, endlessness, enormity,
   enormousness, eternity, exhaustlessness, expanse, forever,
   formidableness, fullness, gigantism, grandeur, grandness,
   great scope, greatness, hugeness, illimitability, immeasurability,
   incalculability, incomprehensibility, inexhaustibility,
   infiniteness, infinitude, infinity, innumerability, intensity,
   interminability, largeness, limitlessness, magnitude,
   measurelessness, might, mightiness, monstrosity, monstrousness,
   muchness, numberlessness, perpetuity, plenitude, power,
   prodigiousness, strength, stupendousness, termlessness,
   tremendousness, universality, unmeasurability, vastness,
   world without end