The Collaborative International Dictionary of English v.0.48:

Immeasurable \Im*meas"ur*a*ble\, a. [Pref. im- not + measurable:
   cf. F. measurable. Cf. Immensurable, Unmeasurable.]
   Incapable of being measured; indefinitely extensive;
   illimitable; immensurable; vast.
   [1913 Webster]

         Of depth immeasurable.                   --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immeasurable
    adj 1: impossible to measure; "unmeasurable reaches of outer
           space" [syn: immeasurable, unmeasurable,
           immensurable, unmeasured] [ant: measurable,
           mensurable]
    2: beyond calculation or measure; "of incalculable value"; "an
       incomputable amount"; "jewels of inestimable value";
       "immeasurable wealth" [syn: incomputable, inestimable,
       immeasurable]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "immeasurable":
   Gargantuan, all-comprehensive, all-inclusive, amplitudinous,
   astronomical, awesome, boundless, bulky, colossal, cosmic,
   countless, endless, enormous, eternal, exhaustless,
   extending everywhere, extensive, galactic, gigantic, great, huge,
   illimitable, illimited, immense, incalculable, incomprehensible,
   indefinite, inestimable, inexhaustible, infinite,
   infinitely continuous, innumerable, interminable, interminate,
   king-size, large, limitless, mammoth, massive, massy, measureless,
   monster, monstrous, monumental, mountainous, no end of, numberless,
   outsize, overgrown, perpetual, prodigious, shoreless, sizable,
   spacious, stupendous, sumless, termless, titanic, tremendous,
   unbounded, uncircumscribed, uncountable, uncounted, unfathomable,
   universal, unlimited, unmeasurable, unmeasured, unnumbered,
   unplumbed, untold, vast, voluminous, weighty, without bound,
   without end, without limit, without measure, without number