Very low-luminosity galaxies in the early universe have observed sizes similar to single star cluster complexes

We compare the sizes and luminosities of 307 faint z=6-8 sources revealed by the Hubble Frontier Fields (HFF) program with sources in the nearby universe. Making use of the latest lensing models and data from the first four HFF clusters with an extensive suite of public lens models, we measure both the sizes and luminosities for 153 z~6, 101 z~7, and 53 z~8 galaxies. The sizes range over more than a decade from ~500 to <50 pc. Extremely small sizes are inferred for many of our lowest luminosity sources, reaching individual sizes as small as 10-30 pc (the smallest is 11(-6)(+28) pc). The uncertainty in these measures ranges from 80 pc for the largest sources to typically about 20 pc for the smallest. Such sizes are smaller than extrapolations of the size-luminosity relation, and expectations for the completeness of our faint samples, suggesting a likely break in the size-luminosity relation at ~-17 mag with size proportional to L**(0.50(-0.11)(+0.10)). The sizes and luminosities of the lowest-luminosity sources 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. Remarkably, our identification of these compact, faint star-forming sources in the z~6-8 universe also allow us to set upper limits on the proto-globular cluster LF at z~6. Comparisons with recent models allow us to rule out (with some caveats) some scenarios for proto-globular cluster formation and set useful upper limits on other less extreme ones. Our results suggest we may be very close to discovering a bona-fide population of forming globular clusters at high redshift.