We present g and z photometry and size estimates for globular clusters (GCs) in the massive Virgo elliptical NGC 4649 (M60) using a five-pointing Hubble Space Telescope/Advanced Camera for Surveys mosaic. The metal-poor GCs show a monotonic negative metallicity gradient of -0.43 ± 0.10 dex per dex in radius over the full radial range of the data, out to ~24 kpc. There is evidence for substantial color substructure among the metal-rich GCs. The metal-poor GCs have typical sizes ~0.4 pc larger than the metal-rich GCs out to large galactocentric distances (gsim 20 kpc), favoring an intrinsic explanation for the size difference rather than projection effects. There is no clear relation between half-light radius and galactocentric distance beyond ~15 kpc, suggesting that the sizes of GCs are not generically set by tidal limitation. Finally, we identify ~20 candidate ultracompact dwarfs that extend down to surprisingly faint absolute magnitudes (Mz ~ -8.5), and may bridge the gap between this class and "extended clusters" in the Local Group. Three of the brighter candidates have published radial velocities and can be confirmed as bona fide ultracompact dwarfs; follow-up spectroscopy will determine the nature of the remainder of the candidates.
