  We present a spectral analysis of NuSTAR and NICER observations of the
luminous, persistently accreting neutron star (NS) low-mass X-ray binary Cygnus
X-2. The data were divided into different branches that the source traces out
on the Z-track of the X-ray color-color diagram; namely the horizontal branch,
normal branch, and the vertex between the two. The X-ray continuum spectrum was
modeled in two different ways that produced a comparable quality fit. The
spectra showed clear evidence of a reflection component in the form of a
broadened Fe K line, as well as a lower energy emission feature near 1 keV
likely due to an ionized plasma located far from the innermost accretion disk.
We account for the reflection spectrum with two independent models (relxillns
and rdblur*rfxconv). The inferred inclination is in agreement with earlier
estimates from optical observations of ellipsoidal light curve modeling
(relxillns: $i=67^{\circ}\pm4^{\circ}$, rdblur*rfxconv:
$i=60^{\circ}\pm10^{\circ}$). The inner disk radius remains close to the NS
($R_{\rm in}\leq1.15\ R_{\mathrm{ISCO}}$) regardless of the source position
along the Z-track or how the 1 keV feature is modeled. Given the optically
determined NS mass of $1.71\pm0.21\ M_{\odot}$, this corresponds to a
conservative upper limit of $R_{\rm in}\leq19.5$ km for $M=1.92\ M_{\odot}$ or
$R_{\rm in}\leq15.3$ km for $M=1.5\ M_{\odot}$. We compare these radius
constraints to those obtained from NS gravitational wave merger events and
recent NICER pulsar light curve modeling measurements.

High Energy Astrophysical Phenomena