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

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