  We present very deep Wide Field Camera 3 (WFC3) photometry of a massive,
compact galaxy located in the Hubble Ultra Deep Field. This quiescent galaxy
has a spectroscopic redshift z=1.91 and has been identified as an extremely
compact galaxy by Daddi et al. 2005. We use new H-F160W imaging data obtained
with Hubble Space Telescope/WFC3 to measure the deconvolved surface brightness
profile to H = 28 mag arcsec**-2. We find that the surface brightness profile
is well approximated by an n=3.7 Sersic profile. Our deconvolved profile is
constructed by a new technique which corrects the best-fit Sersic profile with
the residual of the fit to the observed image. This allows for galaxy profiles
which deviate from a Sersic profile. We determine the effective radius of this
galaxy: r_e=0.42 +- 0.14 kpc in the observed H-F160W-band. We show that this
result is robust to deviations from the Sersic model used in the fit. We test
the sensitivity of our analysis to faint "wings" in the profile using simulated
galaxy images consisting of a bright compact component and a faint extended
component. We find that due to the combination of the WFC3 imaging depth and
our method's sensitivity to extended faint emission we can accurately trace the
intrinsic surface brightness profile, and that we can therefore confidently
rule out the existence of a faint extended envelope around the observed galaxy
down to our surface brightness limit. These results confirm that the galaxy
lies a factor of 10 off from the local mass-size relation.
