  We exploit the superb resolution of the new HST/ACS mosaic image of M51 to
select a large sample of young (< 1 Gyr) star clusters in the spiral disk,
based on their sizes. The image covers the entire spiral disk in B, V, I and
H_alpha, at a resolution of 2 pc per pixel. The surface density distribution of
4357 resolved clusters shows that the clusters are more correlated with clouds
than with stars, and we find a hint of enhanced cluster formation at the
corotation radius. The radius distribution of a sample of 769 clusters with
more accurate radii suggests that young star clusters have a preferred
effective radius of ~3 pc, which is similar to the preferred radius of the much
older GCs. However, in contrast to the GCs, the young clusters in M51 do not
show a relation between radius and galactocentric distance. This means that the
clusters did not form in tidal equilibrium with their host galaxy, nor that
their radius is related to the ambient pressure.
