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We investigate sizes of $z\sim6-8$ dropout galaxies using the complete dataof the Abell 2744 cluster and parallel fields in the Hubble Frontier Fieldsprogram. By directly fitting light profiles of observed galaxies withlensing-distorted S\'ersic profiles on the image plane with the \texttt{glafic}software, we accurately measure intrinsic sizes of 31 $z\sim6-7$ and eight$z\sim8$ galaxies, including those as faint as $M_{\mathrm{UV}}\simeq-16.6$. Wefind that half-light radii $r_\mathrm{e}$ positively correlates with UVluminosity at each redshift, although the correlation is not very tight.Largest ($r_\mathrm{e}>0.8$ kpc) galaxies are mostly red in UV color whilesmallest ($r_\mathrm{e} < 0.08$ kpc) ones tend to be blue. We also find thatgalaxies with multiple cores tend to be brighter. Combined with previousresults at $2.5\lesssim z\lesssim12$, our result confirms that the average$r_{\mathrm{e}}$ of bright ($(0.3-1)L^\star_{z=3}$) galaxies scales as$r_{\mathrm{e}}\propto(1+z)^{-m}$ with $m=1.24\pm0.1$. We find that the ratioof $r_\mathrm{e}$ to virial radius is virtually constant at $3.3\pm0.1\%$ overa wide redshift range, where the virial radii of hosting dark matter halos arederived based on the abundance matching. This constant ratio is consistent withthe disk formation model by Mo et al. (1998) with $j_\mathrm{d}\simm_\mathrm{d}$, where $j_\mathrm{d}$ and $m_\mathrm{d}$ are the fractions of theangular momentum and mass within halos confined in the disks. A comparison withvarious types of local galaxies indicates that our galaxies are most similar tocircumnuclear star-forming regions of barred galaxies in the sense that asizable amount of stars are forming in a very small area.

The sizes of $z\sim6-8$ lensed galaxies from the Hubble Frontier Fields Abell 2744 data