  Deep spectroscopy of the two mJy radio galaxies LBDS 53W069 and 53W091 has
previously shown them to have old (>3 Gyr) stellar populations at z=1.5. Here
we present the results of HST observations of these galaxies. 53W069 has an
r^1/4 profile in both the F814W & F110W data with an effective radius of 0.30
arcsec (2.7 kpc). The restframe U-B colour gradient requires a stellar
population of super-solar (3Z_sun) metallicity that formed on a very short
timescale at high redshift (z>5). 53W091 has a regular r^1/4 profile in F110W
with an effective radius of 0.32 arcsec (2.9 kpc). The F814W profile is more
extended and is consistent with a blue exponential disk that contributes 20% of
the flux within r_e. The restframe U-B colour gradient is significantly larger
than that observed in field ellipticals at z<1, implying a stellar population
of mixed metallicity (1-3Z_sun) that formed in a high-redshift rapid burst.
  We have compared these two LBDS radio galaxies with the Kormendy relation of
ten 3CR radio galaxies at z=0.8. The LBDS galaxies follow the same relation as
the more radio-luminous 3CR galaxies, assuming passive evolution of their
stellar populations, although they are smaller than the 3CR galaxies whose mean
effective radius is 12 kpc. Their sizes and radio luminosities are consistent
with scaling relations applied to the 3CR galaxies, in which both radio power
and effective radius scale with galaxy mass. Compared with a sample of z=0.4
cluster ellipticals, 53W069 & 53W091 lie well within the scatter of the
Kormendy relation. We conclude that the hosts of these mJy radio sources at
z=1.5 are passively-evolving elliptical galaxies that will evolve into ordinary
L* ellipticals by the present day. (Abridged)
