  We present the intrinsic and observed sizes of galaxies at $z\geq5$ in the
First Light And Reionisation Epoch Simulations (FLARES). We employ the large
effective volume of FLARES to produce a sizeable sample of high redshift
galaxies with intrinsic and observed luminosities and half light radii in a
range of rest frame UV and visual photometric bands. This sample contains a
significant number of intrinsically ultra-compact galaxies in the far-UV (1500
angstrom), leading to a negative intrinsic far-UV size-luminosity relation.
However, after the inclusion of the effects of dust these same compact galaxies
exhibit observed sizes that are as much as 50 times larger than those measured
from the intrinsic emission, and broadly agree with a range of observational
samples. This increase in size is driven by the concentration of dust in the
core of galaxies, heavily attenuating the intrinsically brightest regions. At
fixed luminosity we find a galaxy size redshift evolution with a slope of
$m=1.21-1.87$ depending on the luminosity sample in question, and we
demonstrate the wavelength dependence of the size-luminosity relation which
will soon be probed by the Webb Space Telescope.
