Low-luminosity galaxies in the early universe have observed sizes
  similar to star cluster complexes
  We compare the sizes and luminosities of faint $z=6$-8 galaxies magnified by
the Hubble Frontier Fields (HFF) clusters with star-forming regions, as well as
more evolved objects, in the nearby universe. Our high-redshift comparison
sample includes 333 z=6-8 galaxies, for which size measurements were made as
part of a companion study where lensing magnifications were estimated from
various public models. Accurate size measurements for these sources are
complicated by the lens model uncertainties, but other results and arguments
suggest that faint galaxies are small, as discussed in a companion study. The
measured sizes for sources in our comparison sample range from <50 pc to ~500
pc. For many of the lowest luminosity sources, extremely small sizes are
inferred, reaching individual sizes as small as 10-30 pc, with several sources
in the 10-15 pc range with our conservative magnification limits. The sizes and
luminosities are similar to those of single star cluster complexes like 30
Doradus in the lower-redshift universe and -- in a few cases -- super star
clusters. The identification of these compact, faint star-forming sources in
the z~6-8 universe also allows us to set upper limits on the proto-globular
cluster LF at z~6. By comparisons of the counts and sizes with recent models,
we rule out (with some caveats) proto-globular cluster formation scenarios
favoring substantial (xi=10) post-formation mass loss and set useful upper
limits on others. Our size results suggest we may be very close to discovering
a bona-fide population of forming globular clusters at high redshift.
