The Collaborative International Dictionary of English v.0.48:

Stupendous \Stu*pen"dous\, a. [L. stupendus astonishing, p.
   future pass. of stupere to be astonished at. Cf. Stupid.]
   Astonishing; wonderful; amazing; especially, astonishing in
   magnitude or elevation; as, a stupendous pile. "A stupendous
   sum." --Macaulay.
   [1913 Webster]

         All are but parts of one stupendous whole. --Pope.
   [1913 Webster] -- Stu*pen"dous*ly, adv. --
   Stu*pen"dous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stupendous
    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:

96 Moby Thesaurus words for "stupendous":
   Atlantean, Brobdingnagian, Cyclopean, Gargantuan, Herculean,
   Homeric, abysmal, amazing, amplitudinous, astonishing, astounding,
   astronomic, astronomical, awesome, beguiling, bewildering,
   boundless, bulky, colossal, cosmic, cracking, elephantine,
   enigmatic, enormous, epic, exceptional, extensive, extraordinary,
   fabulous, fantastic, fascinating, galactic, giant, giantlike,
   gigantic, heroic, huge, immeasurable, immense, incomprehensible,
   inconceivable, incredible, indescribable, ineffable, infinite,
   jumbo, king-size, large, legendary, mammoth, marvelous, massive,
   massy, mighty, miraculous, monster, monstrous, monumental, mortal,
   mountainous, mythical, noteworthy, outlandish, outsize, overgrown,
   passing strange, phenomenal, portentous, prodigious, profound,
   puzzling, rare, remarkable, sensational, sizable, spacious,
   spectacular, staggering, strange, striking, stupefying, titanic,
   towering, tremendous, unexampled, unheard-of, unimaginable, unique,
   unparalleled, unprecedented, unspeakable, vast, voluminous,
   weighty, wonderful, wondrous