The size-luminosity relation of lensed galaxies at $z=6-9$ in the Hubble
  Frontier Fields
  We measure the size-luminosity relation of photometrically-selected galaxies
within the redshift range $z\sim6-9$, using galaxies lensed by six foreground
Hubble Frontier Fields (HFF) clusters. The power afforded by strong
gravitational lensing allows us to observe fainter and smaller galaxies than in
blank fields. We select our sample of galaxies and obtain their properties,
e.g., redshift, magnitude, from the photometrically-derived ASTRODEEP
catalogues. The intrinsic size is measured with the Lenstruction software, and
completeness maps are created as a function of size and luminosity via the
GLACiAR2 software. We perform a Bayesian analysis to estimate the intrinsic and
incompleteness-corrected size-luminosity distribution, with parameterization
$r_e \propto L^\beta$. We find slopes of $\beta\sim0.48\pm0.08$ at $z\sim6-7$
and $\beta\sim0.68\pm0.14$ at $z\sim8.5$, adopting the Bradac lens model. The
slope derived by lensed galaxies is steeper than that obtained in blank fields
and is consistent with other independent determinations of the size-luminosity
relation from the HFF dataset. We also investigate the systematic uncertainties
correlated with the choice of lens models, finding that the slopes of
size-luminosity relations derived from different models are consistent with
each other, i.e. the modeling errors are not a significant source of
uncertainty in the size-luminosity relation.
