  We present visible and near-infrared photometric and spectroscopic
observations of interstellar object 2I/Borisov taken from 2019 September 10 to
2019 November 29 using the GROWTH, the APO ARC 3.5 m and the NASA/IRTF 3.0 m
combined with post and pre-discovery observations of 2I obtained by ZTF from
2019 March 17 to 2019 May 5. Comparison with imaging of distant Solar System
comets shows an object very similar to mildly active Solar System comets with
an out-gassing rate of $\sim$10$^{27}$ mol/sec. The photometry, taken in
filters spanning the visible and NIR range shows a gradual brightening trend of
$\sim0.03$ mags/day since 2019 September 10 UTC for a reddish object becoming
neutral in the NIR. The lightcurve from recent and pre-discovery data reveals a
brightness trend suggesting the recent onset of significant H$_2$O sublimation
with the comet being active with super volatiles such as CO at heliocentric
distances $>$6 au consistent with its extended morphology. Using the advanced
capability to significantly reduce the scattered light from the coma enabled by
high-resolution NIR images from Keck adaptive optics taken on 2019 October 04,
we estimate a diameter of 2I's nucleus of $\lesssim$1.4 km. We use the size
estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the
ISO size-distribution resulting in a slope of $\sim$3.4$\pm$1.2, similar to
Solar System comets and bodies produced from collisional equilibrium.
