  A search for stellar clusters has been carried out in 18 nearby spiral
galaxies, using HST/WFPC2 archive images. All of the galaxies have previously
been imaged from the ground in UBVI. A catalogue of structural parameters,
photometry and comments based on visual inspection of the clusters is compiled
and used to investigate correlations between cluster structure, environment,
age and mass. Least-squares fits to the data suggest correlations between both
the FWHM and half-light radius (Reff) of the clusters and their masses (M) at
about the 3-sigma level. Although both relations show a large scatter, the fits
have substantially shallower slopes than for a constant-density relation.
However, many of the youngest clusters have extended halos which make the Reff
determinations uncertain. There is no evidence for galaxy-to-galaxy variations
in the mean cluster sizes. In particular, the mean sizes do not appear to
depend on the host galaxy star formation rate surface density. Many of the
youngest objects (age < 10 Myr) are located in strongly crowded regions, and
about 1/3-1/2 of them are double or multiple sources. The HST images are also
used to check the nature of cluster candidates identified in a previous
ground-based survey. The contamination rate in the ground-based sample is
generally less than about 20%, but some cluster identifications remain
ambiguous because of crowding even with HST imaging, especially for the
youngest objects.
