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We study the morphologies and sizes of galaxies at z>5 using high-resolutioncosmological zoom-in simulations from the Feedback In Realistic Environmentsproject. The galaxies show a variety of morphologies, from compact to clumpy toirregular. The simulated galaxies have more extended morphologies and largersizes when measured using rest-frame optical B-band light than rest-frame UVlight; sizes measured from stellar mass surface density are even larger. The UVmorphologies are usually dominated by several small, bright young stellarclumps that are not always associated with significant stellar mass. The B-bandlight traces stellar mass better than the UV, but it can also be biased by thebright clumps. At all redshifts, galaxy size correlates with stellarmass/luminosity with large scatter. The half-light radii range from 0.01 to 0.2arcsec (0.05-1 kpc physical) at fixed magnitude. At z>5, the size of galaxiesat fixed stellar mass/luminosity evolves as (1+z)^{-m}, with m~1-2. Forgalaxies less massive than M_star~10^8 M_sun, the ratio of the half-mass radiusto the halo virial radius is ~10% and does not evolve significantly at z=5-10;this ratio is typically 1-5% for more massive galaxies. A galaxy's "observed"size decreases dramatically at shallower surface brightness limits. This effectmay account for the extremely small sizes of z>5 galaxies measured in theHubble Frontier Fields. We provide predictions for the cumulative lightdistribution as a function of surface brightness for typical galaxies at z=6.

Simulating galaxies in the reionization era with FIRE-2: morphologies and sizes