  Microlensing perturbations to the magnification of gravitationally lensed
quasar images are dependent on the angular size of the quasar. If quasar
variability at visible wavelengths is caused by a change in the area of the
accretion disk, it will affect the microlensing magnification. We derive the
expected signal, assuming that the luminosity scales with some power of the
disk area, and estimate its amplitude using simulations. We discuss the
prospects for detecting the effect in real-world data and for using it to
estimate the logarithmic slope of the luminosity's dependence on disk area.
Such an estimate would provide a direct test of the standard thin accretion
disk model. We tried fitting six seasons of the light curves of the lensed
quasar HE 0435-1223 including this effect as a modification to the Kochanek et
al. (2006) approach to estimating time delays. We find a dramatic improvement
in the goodness of fit and relatively plausible parameters, but a robust
estimate will require a full numerical calculation in order to correctly model
the strong correlations between the structure of the microlensing magnification
patterns and the magnitude of the effect. We also comment briefly on the effect
of this phenomenon for the stability of time delay estimates.
