  We investigate the structural properties of nuclear star clusters in
late-type spiral galaxies. More specifically, we fit analytical models to HST
images of 39 nuclear clusters in order to determine their effective radii after
correction for the instrumental point spread function. We use the results of
this analysis to compare the luminosities and sizes of nuclear star clusters to
those of other ellipsoidal stellar systems, in particular the Milky Way
globular clusters. Our nuclear clusters have a median effective radius of r_e =
3.5 pc, with 50% of the sample falling between 2.4 pc < r_e < 5.0 pc. This
narrow size distribution is statistically indistinguishable from that of
Galactic globular clusters, even though the nuclear clusters are on average 4
magnitudes brighter than the old globulars. We discuss some possible
interpretations of this result. From a comparison of nuclear cluster
luminosities with various properties of their host galaxies, we confirm that
more luminous galaxies harbor more luminous nuclear clusters. It remains
unclear whether this correlation mainly reflects the influence of galaxy size,
mass, and/or star formation rate. Since the brighter galaxies in our sample
typically have stellar disks with a higher central surface brightness, nuclear
cluster luminosity also correlates with this property of their hosts. On the
other hand, we find no evidence for a correlation between the presence of a
nuclear star cluster and the presence of a large-scale stellar bar.
