Galaxy size. Left: Galaxy size as a function of halo mass and redshift as predicted by FFB disk model assuming an interim value of ϵ_max = 0.5. Right: Predicted galaxy half-mass radius as a function of redshift for galaxies at the FFB threshold. The estimates by the two limiting FFB models of shell (blue) and disk (orange), from Eqs. (31) and (33), are pretty similar, predicting roughly $R_{\text{e}}\sim 0.3\text{kpc}{(1+z)}_{10}^{-2.5}$. The color bands crudely indicate the uncertainty considering the scatter in the spin parameter. The dependence on ϵ_max is weak. Shown for comparison are tentative JWST observations (Baggen et al. 2023; Robertson et al. 2023b; Tacchella et al. 2023a; Finkelstein et al. 2023b; Casey et al. 2024; Sun et al. 2024; McGrath et al., in prep.). The model provides a rough but reasonable estimate of the radius and effectively captures the observed decline of radius with redshift.