We present a study of the effective ( half-light ) radii and other structural properties of a systematically selected sample of young , massive star clusters ( YMCs , \geq 5 \times 10 ^ { 3 } M _ { \odot } and \leq 200 Myr ) in two nearby spiral galaxies , NGC 628 and NGC 1313 .
We use Hubble Space Telescope WFC3/UVIS and archival ACS/WFC data obtained by the Legacy Extragalactic UV Survey ( LEGUS ) , an HST Treasury Program .
We measure effective radii with GALFIT , a two-dimensional image-fitting package , and with a new technique to estimate effective radii from the concentration index ( CI ) of observed clusters .
The distribution of effective radii from both techniques spans \sim 0.5–10 pc and peaks at 2-3 pc for both galaxies .
We find slight positive correlations between effective radius and cluster age in both galaxies , but no significant relationship between effective radius and galactocentric distance .
Clusters in NGC 1313 display a mild increase in effective radius with cluster mass , but the trend disappears when the sample is divided into age bins .
We show that the vast majority of the clusters in both galaxies are much older than their dynamical times , suggesting they are gravitationally bound objects .
We find that about half of the clusters in NGC 628 are underfilling their Roche lobes , based on their Jacobi radii .
Our results suggest that the young , massive clusters in NGC 628 and NGC 1313 are expanding due to stellar mass loss or two-body relaxation and are not significantly influenced by the tidal fields of their host galaxies .