  We derive structural parameters for ~2000 globular clusters in the giant
Virgo elliptical M87 using extremely deep Hubble Space Telescope images in
F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes
(half-light radii r_h) and ellipticities are determined from PSF-convolved
King-model profile fitting. We find that the r_h distribution closely resembles
the inner Milky Way clusters, peaking at r_h~2.5 pc and with virtually no
clusters more compact than r_h ~ 1 pc. The metal-poor clusters have on average
an r_h 24% larger than the metal-rich ones. The cluster scale size shows a
gradual and noticeable increase with galactocentric distance. Clusters are very
slightly larger in the bluer waveband V a possible hint that we may be
beginning to see the effects of mass segregation within the clusters. We also
derived a color magnitude diagram for the M87 globular cluster system which
show a striking bimodal distribution.
