  High precision mapping of H2O megamaser emission from active galaxies has
revealed more than a dozen Keplerian H2O maser disks, which enable a ~4%
uncertainty estimate of the Hubble constant as well as providing accurate
masses for the central black holes. These disks often have well-defined inner
and outer boundaries of maser emission on sub-parsec scales. In order to better
understand the physical conditions that determine the inner and outer radii of
a maser disk, we examine the distributions of gas density and X-ray heating
rate in a warped molecular disk described by a power-law surface density
profile. For a suitable choice of the disk mass, we find that the outer radius
R_out of the maser disk predicted from our model can match the observed value,
with R_out mainly determined by the maximum heating rate or the minimum density
for efficient maser action, depending on the combination of the Eddington
ratio, black hole mass, and disk mass. Our analysis also indicates that the
inner radius for maser action is comparable to the dust sublimation radius,
suggesting that dust may play a role in determining the inner radius of a maser
disk. Finally, our model predicts that H2O gigamaser disks could exist at the
centers of high-z quasars, with disk sizes of >~ 10-30 pc.
