The Collaborative International Dictionary of English v.0.48:

Colossal \Co*los"sal\, a. [Cf. F. colossal, L. colosseus. See
   Colossus.]
   1. Of enormous size; gigantic; huge; as, a colossal statue.
      "A colossal stride." --Motley.
      [1913 Webster]

   2. (Sculpture & Painting) Of a size larger than heroic. See
      Heroic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

colossal
    adj 1: so great in size or force or extent as to elicit awe;
           "colossal crumbling ruins of an ancient temple"; "has a
           colossal nerve"; "a prodigious storm"; "a stupendous
           field of grass"; "stupendous demand" [syn: colossal,
           prodigious, stupendous]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "colossal":
   Atlantean, Brobdingnagian, Cyclopean, Gargantuan, Herculean,
   Homeric, Olympian, abysmal, aerial, airy, altitudinous,
   amplitudinous, ascending, aspiring, astronomic, astronomical,
   awe-inspiring, awesome, boundless, bulky, cosmic, cyclopean,
   dominating, elephantine, elevated, eminent, enormous, epic,
   ethereal, exalted, extensive, extraordinary, galactic, gangling,
   gangly, gargantuan, giant, giantlike, gigantic, haughty, heroic,
   high, high-pitched, high-reaching, high-set, high-up, huge,
   immeasurable, immense, incredible, infinite, jumbo, king-size,
   lank, lanky, large, leggy, lengthy, lofty, long, long-legged,
   mammoth, massive, massy, mighty, monster, monstrous, monumental,
   mountainous, mounting, outsize, outtopping, overgrown, overlooking,
   overtopping, overwhelming, prodigious, profound, prominent, rangy,
   sizable, soaring, spacious, spectacular, spiring, staggering,
   statuesque, steep, stupendous, sublime, superlative, supernal,
   tall, titanic, topless, toplofty, topping, towering, towery,
   tremendous, unbelievable, uplifted, upreared, vast, voluminous,
   weighty, wonderful