Since the amplitude of microlensing variability depends on a source size the monitoring of gravitationally lensed quasars can produce the valuable information about the accretion disk size. Comparison of standard deviations in two wavebands yields useful constraints from quite moderate number of multicolor observations.

Existing optical VRI monitoring data for the quadruple gravitationally lensed quasar Q2237+ 0305 allow to obtain the source size ratio in two bands, V and R or V and I, as a function of size in the V band. Agreement with the standard accretion disk theory which predicts the dependency of a source size on wavelength ∼ λ4/3 can be achieved only if the quasar size in V band is comparable with the Einstein radius (≃ 1017 cm).