Radii of Young Star Clusters in Nearby Galaxies
  We measure the projected half-light radii of young star clusters in 31
galaxies from the Legacy Extragalactic UV Survey (LEGUS). We implement a custom
pipeline specifically designed to be robust against contamination, which allows
us to measure radii for 6097 clusters. This is the largest sample of young star
cluster radii currently available. We find that most (but not all) galaxies
share a common cluster radius distribution, with the peak at around 3 pc. We
find a clear mass-radius relation of the form $R_{\mathrm{eff}} \propto
M^{0.24}$. This relation is present at all cluster ages younger than 1 Gyr, but
with a shallower slope for clusters younger than 10 Myr. We present simple toy
models to interpret these age trends, finding that high-mass clusters are more
likely to be not tidally limited and expand. We also find that most clusters in
LEGUS are gravitationally bound, especially at older ages or higher masses.
Lastly, we present the cluster density and surface density distributions,
finding a large scatter that appears to decrease with cluster age. The youngest
clusters have a typical surface density of 100 $M_\odot$ pc$^{-2}$.
