  1I/`Oumuamua is the first confirmed interstellar body in our Solar System.
Here we report on observations of `Oumuamua made with the Spitzer Space
Telescope on 2017 November 21--22 (UT). We integrated for 30.2~hours at 4.5
micron (IRAC channel 2). We did not detect the object and place an upper limit
on the flux of 0.3 uJy (3sigma). This implies an effective spherical diameter
less than [98, 140, 440] meters and albedo greater than [0.2, 0.1, 0.01] under
the assumption of low, middle, or high thermal beaming parameter eta,
respectively. With an aspect ratio for `Oumuamua of 6:1, these results
correspond to dimensions of [240:40, 341:57, 1080:180] meters, respectively. We
place upper limits on the amount of dust, CO, and CO2 coming from this object
that are lower than previous results; we are unable to constrain the production
of other gas species. Both our size and outgassing limits are important because
`Oumuamua's trajectory shows non-gravitational accelerations that are sensitive
to size and mass and presumably caused by gas emission. We suggest that
`Oumuamua may have experienced low-level post-perihelion volatile emission that
produced a fresh, bright, icy mantle. This model is consistent with the
expected eta value and implied high albedo value for this solution, but, given
our strict limits on CO and CO2, requires another gas species --- probably H2O
--- to explain the observed non-gravitational acceleration. Our results extend
the mystery of `Oumuamua's origin and evolution.
