From "Usage and Abusage: A Guide to Good English" By Eric
Partridge: Unmeasurable is preferred in the two literal senses
'incapable of being measured, on account of great size, extent or
amount, in reference to material things, to dimensions, to time' as
in 'The tower was of an unmeasurable height' and 'not admitting of
- insusceptible of - measurement' as in 'The church is unmeasurable
by foot-rule' (OED). But in the sense of 'too great for
measurement, immense', immeasurable is preferrable, as in
'immeasuable ambition', 'the immeasurable grace of God.'