The Collaborative International Dictionary of English v.0.48:

Monumental \Mon`u*men"tal\, a. [L. monumentalis: cf. F.
   monumental.]
   1. Of, pertaining to, or suitable for, a monument; as, a
      monumental inscription.
      [1913 Webster]

   2. Serving as a monument; memorial; preserving memory. "Of
      pine, or monumental oak." --Milton.
      [1913 Webster]

            A work outlasting monumental brass.   --Pope.
      [1913 Webster]

   3. Of lasting significance; as, a monumental work of
      literature; a monumental accomplishment.
      [PJC]

   4. Exceptionally large in quantity, quality, or degree; as, a
      monumental amount of work to be done
      [PJC]

WordNet (r) 3.0 (2006):

monumental
    adj 1: relating or belonging to or serving as a monument; "the
           use of the arch in monumental architecture"; "monumental
           sculptures"
    2: of outstanding significance; "Einstein's monumental
       contributions to physics"
    3: imposing in size or bulk or solidity; "massive oak doors";
       "Moore's massive sculptures"; "the monolithic proportions of
       Stalinist architecture"; "a monumental scale" [syn:
       massive, monolithic, monumental]

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "monumental":
   Atlantean, Brobdingnagian, Cyclopean, Gargantuan, Herculean,
   Homeric, Olympian, abject, abysmal, aerial, airy, altitudinous,
   amplitudinous, ascending, aspiring, astronomic, astronomical,
   awe-inspiring, awesome, awful, boundless, bulky, calamitous,
   catastrophic, classic, colossal, commemorative, cosmic, cracking,
   dominating, egregious, elephantine, elevated, eminent, enduring,
   enormous, epic, ethereal, exalted, extensive, fantastic, galactic,
   giant, giantlike, gigantic, glorious, grand, haughty, heroic, high,
   high-pitched, high-reaching, high-set, high-up, historic, huge,
   immeasurable, immense, impressive, infinite, jumbo, king-size,
   large, lasting, lofty, magnificent, mammoth, marmoreal, massive,
   massy, memorable, memorial, mighty, monster, monstrous, mortal,
   mountainous, mounting, notable, noteworthy, outsize, outstanding,
   outtopping, overgrown, overlooking, overtopping, overwhelming,
   permanent, prodigious, profound, prominent, sculptitory,
   sculptural, sculpturesque, significant, sizable, soaring, spacious,
   spectacular, spiring, staggering, statuary, statuelike, statuesque,
   steep, striking, stupendous, sublime, superlative, supernal,
   terrible, titanic, topless, toplofty, topping, towering, towery,
   tremendous, unbelievable, unforgettable, unforgivable, uplifted,
   upreared, vast, voluminous, weighty, whopping, wonderful,
   wondrous