  We present a catalog of about 25,000 images of massive ($M_{\star} \ge 10^9
M_{\odot}$) galaxies at redshift $3 \leq z \leq 6$ from the TNG50 cosmological
simulation, tailored for observations at multiple wavelengths carried out with
JWST. The synthetic images were created with the SKIRT radiative transfer code,
including the effects of dust attenuation and scattering. The noiseless images
were processed with the mirage simulator to mimic the Near Infrared Camera
(NIRCam) observational strategy (e.g., noise, dithering pattern, etc.) of the
Cosmic Evolution Early Release Science (CEERS) survey. In this paper, we
analyze the predictions of the TNG50 simulation for the size evolution of
galaxies at $3 \leq z \leq 6$ and the expectations for CEERS to probe that
evolution. In particular, we investigate how sizes depend on wavelength,
redshift, mass, and angular resolution of the images. We find that the
effective radius accurately describes the three-dimensional half-mass radius of
TNG50 galaxies. Sizes observed at 2~$\mu$m are consistent with those measured
at 3.56~$\mu$m at all redshifts and masses. At all masses, the population of
higher-$z$ galaxies is more compact than their lower-$z$ counterparts. However,
the intrinsic sizes are smaller than the mock observed sizes for the most
massive galaxies, especially at $z \lesssim 4$. This discrepancy between the
mass and light distribution may point to a transition in the galaxy morphology
at $z$=4-5, where massive compact systems start to develop more extended
stellar structures.
