  We use the microlensing variability observed for eleven gravitationally
lensed quasars to show that the accretion disk size at a rest-frame wavelength
of 2500 Angstroms is related to the black hole mass by
log(R_{2500}/cm)=(15.78\pm0.12) + (0.80\pm0.17)\log(M_BH/10^9M_sun). This
scaling is consistent with the expectation from thin disk theory (R ~
M_BH^{2/3}), but when interpreted in terms of the standard thin disk model (T ~
R^{-3/4}), it implies that black holes radiate with very low efficiency,
log(eta) = -1.77\pm0.29 + log(L/L_E) where eta=L/(Mdot*c^2). Only by making the
maximum reasonable shifts in the average inclination, Eddington factors and
black hole masses can we raise the efficiency estimate to be marginally
consistent with typical efficiency estimates (eta ~ 10%). With one exception,
these sizes are larger by a factor of ~4 than the size needed to produce the
observed 0.8 micron quasar flux by thermal radiation from a thin disk with the
same T ~ R^{-3/4} temperature profile. While scattering a significant fraction
of the disk emission on large scales or including a large fraction of
contaminating line emission can reduce the size discrepancy, resolving it also
appears to require that accretion disks have flatter temperature/surface
brightness profiles.
