  In Paper-I we developed a two-phase model to connect dynamically hot galaxies
(such as ellipticals and bulges) with the formation of self-gravitating gas
clouds (SGCs) associated with the fast assembly of dark matter halos. Here we
explore the implications of the model for the size-stellar mass relation of
dynamically hot galaxies. Star-forming sub-clouds resulting from the
fragmentation of the turbulent SGC inherit its spatial structure and dynamical
hotness, producing a `homologous' relation, $r_{\rm f}\approx\, 100 r_{\rm
bulge}$, between the size of a dynamically hot galaxy ($r_{\rm bulge}$) and
that of its host halo assembled in the fast regime ($r_{\rm f}$), independent
of redshift and halo mass. This relation is preserved by the `dry' expansion
driven by dynamical heating when a galaxy becomes gas-poor due to inefficient
cooling, and is frozen due to the stop of bulge growth during the slow assembly
regime of the halo. The size-stellar mass relation is thus a simple combination
of the galaxy-halo homology and the non-linear stellar mass-halo mass relation.
Using a set of halo assembly histories we reproduce all properties in the
observed size-mass relation of dynamically hot galaxies, including the
flattening in the low-mass end and the upturn in the massive end. The
prediction matches observational data currently available to $z \approx 4$, and
can be tested in the future at higher $z$. Our results indicate that the sizes
of dynamically hot galaxies are produced by the dissipation and collapse of gas
in halos to establish SGCs in which stars form.
