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The galaxy size-stellar mass and central surface density-stellar massrelationships are observational constraints on galaxy formation models.However, inferring the physical size of a galaxy from observed stellar emissionis non-trivial due to various observational effects. Consequently,forward-modeling light-based sizes from simulations is desirable. In this work,we use the {\skirt} dust radiative transfer code to generate syntheticobservations of massive galaxies ($M_{*}\sim10^{11}\,\rm{M_{\odot}}$ at $z=2$,hosted by haloes of mass $M_{\rm{halo}}\sim10^{12.5}\,\rm{M_{\odot}}$) fromhigh-resolution cosmological zoom-in simulations that form part of the FeedbackIn Realistic Environments (FIRE) project. The simulations used in this paperinclude explicit stellar feedback but no active galactic nucleus (AGN)feedback. From each mock observation, we infer the effective radius ($R_e$), aswell as the stellar mass surface density within this radius and within$1\,\rm{kpc}$ ($\Sigma_e$ and $\Sigma_1$, respectively). We first investigatehow well the intrinsic half-mass radius and stellar mass surface density can beinferred from observables. The predicted sizes and surface densities are withina factor of two of the intrinsic values. We then compare our predictions to theobserved size-mass relationship and the $\Sigma_1-M_\star$ and$\Sigma_e-M_\star$ relationships. At $z\gtrsim2$, the simulated massivegalaxies are in general agreement with observational scaling relations. At$z\lesssim2$, they evolve to become too compact but still star-forming, in thestellar mass and redshift regime where many of them should be quenched. Ourresults suggest that some additional source of feedback, such as AGN drivenoutflows, is necessary in order to decrease the central densities of thesimulated massive galaxies to bring them into agreement with observations at$z\lesssim2$.

Realistic mock observations of the sizes and stellar mass surface densities of massive galaxies in FIRE-2 zoom-in simulations