  We use Hubble Space Telescope imaging to study the structural properties of
ten of the most massive ($M \geq 10^{11.25}$ Msun) quiescent galaxies (QGs) in
the UKIDSS UDS at $2.5<z<3.0$. The low spatial density of these galaxies
required targeted WFC3 $H_{160}$ imaging, as such systems are rare in existing
surveys like CANDELS. We fit Sersic models to the 2D light profiles and find
that the median half-light radius is $R_e \sim 3$ kpc, a factor of $\sim 3$
smaller than QGs with similar masses at $z \sim 0$. Complementing our sample
with similarly massive QGs at lower redshifts, we find that the median size
evolves as $R_e \propto H(z)^{-0.85 \pm 0.12}$ (or alternatively, $R_e \propto
(1+z)^{-0.90 \pm 0.12}$). This rate of evolution is slower than that for lower
mass QGs. When compared to low redshift QGs, the axis ratio distribution for
our high redshift massive QG sample is most consistent with those in which
spheroids are dominant. These observations point to earlier size growth among
massive QGs that also resulted in spheroidal systems. Finally, we measured
residual-corrected surface brightness profiles for our sample. These show that
the Sersic parameterization is generally representative out to several
effective radii and does not miss excess low surface brightness light. The
sizes inferred from the light profiles therefore confirm the compactness of
these most massive high redshift QGs.
