Angular Sizes of $\mu$Jy Radio Sources
  We made two new sensitive (rms noise sigma_n ~ 1 microJy/beam) high
resolution (theta = 3.0" and theta = 0.66" FWHM) S--band (2 < nu < 4 GHz)
images covering a single JVLA primary beam (FWHM ~ 14') centered on J2000 RA =
10 46, Dec = 59 01 in the Lockman Hole. These images yielded a catalog of 792
radio sources, 97.7 +/- 0.8% of which have infrared counterparts stronger than
S ~ 2 microJy at lambda = 4.5 micron. About 91% of the radio sources found in
our previously published, comparably sensitive low resolution (theta = 8" FWHM)
image covering the same area were also detected at 0.66" resolution, so most
radio sources with S_3GHz >~ 5 microJy have angular structure phi <~ 0.66". The
ratios of peak brightness in the 0.66" and 3" images have a distribution
indicating that most microJy radio sources are quite compact, with a median
Gaussian angular diameter <phi> = 0.3" +/- 0.1" FWHM and an rms scatter
sigma_phi <~ 0.3" of individual sizes. Most of our microJy radio sources obey
the tight far-infrared/radio correlation, indicating that they are powered by
star formation. The median effective angular radius enclosing half the light
emitted by an exponential disk is <rho_e> ~ <phi>/2.43 ~ 0.12", so the median
effective radius of star-forming galaxies at redshifts z~1 is <r_e> ~ 1.0 kpc.
