  2I/Borisov is the second interstellar object (ISO) after 'Oumuamua (Meech et
al. 2017), but differs from 'Oumuamua drastically with its extensive cometary
activity. A key ingredient to understand the nature of this comet is its size.
However, due to its cometary activity and extended coma in the optical, only
rough estimates and upper limits can be made for 2I/Borisov, ranging in a wide
spread from 0.7 to 3.8 km (Guzik et al. 2019; Fitzsimmons et al. 2019; Jewitt,
& Luu 2019; Bolin et al. 2019). It has been shown that observations at longer
wavelengths (i.e. infrared) are less susceptible to the effects of coma, and
can provide a better estimate of the size of the comet nucleus (see, e.g.,
Fern\'andez et al. 2013; Bauer et al. 2017). Here we present an estimate of the
nucleus of 2I/Borisov from infrared observations by FLAMINGOS-2 on-board the
Gemini South telescope (under Fast Turnaround program GS-2019B-FT-207), and
infer a comet nucleus size of 1.5 km, comparable to but more stringent than the
estimate from Keck AO imaging by Bolin et al. (2019).
