The Collaborative International Dictionary of English v.0.48:

Immensurable \Im*men"su*ra*ble\, a. [Pref. im- not + L.
   mensurabilis measurable: cf. F. immensurable. Cf.
   Immeasurable.]
   Immeasurable.
   [1913 Webster]

         What an immensurable space is the firmament. --Derham.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immensurable
    adj 1: impossible to measure; "unmeasurable reaches of outer
           space" [syn: immeasurable, unmeasurable,
           immensurable, unmeasured] [ant: measurable,
           mensurable]