As part of the Megamaser Cosmology Project, we present VLBI maps of nuclear water masers toward five galaxies. The masers originate in sub-parsec circumnuclear disks. For three of the galaxies, we fit Keplerian rotation curves to estimate their supermassive black hole (SMBH) masses, and determine (2.9+/-0.3)x10^6^M_{sun}_ for J0437+2456, (1.7+/-0.1)x10^7^M_{sun}_ for ESO 558-G009, and (1.1+/-0.2)x10^7^M_{sun}_ for NGC 5495. In the other two galaxies, Mrk 1029 and NGC 1320, the geometry and dynamics are more complicated and preclude robust black hole mass estimates. Including our new results, we compiled a list of 15 VLBI-confirmed disk maser galaxies with robust SMBH mass measurements. With this sample, we confirm the empirical relation of R_out_{propto}0.3M_SMBH_ reported in Wardle & Yusef-Zadeh (2012ApJ...750L..38W). We also find a tentative correlation between maser disk outer radii and Wide-Field Infrared Survey Explorer luminosity. We find no correlations of maser disk size with X-ray 2-10keV luminosity or [OIII] luminosity.