  We aim to use signatures of microlensing induced by stars in the foreground
lens galaxy to infer the size of the accretion disk in the gravitationally
lensed quasar Q 0957+561. The long-term photometric monitoring of this system
(which so far has provided the longest available light curves of a
gravitational lens system) permits us to evaluate the impact of uncertainties
on our recently developed method (controlled by the distance between the
modeled and the experimental magnitude difference histograms between two lensed
images), and thus to test the robustness of microlensing-based disk-size
estimates. We analyzed the well-sampled 21-year GLENDAMA optical light curves
of the double-lensed quasar and studied the intrinsic and extrinsic continuum
variations. Using accurate measurements for the time delay between the images A
and B, we modeled and removed the intrinsic quasar variability, and from the
statistics of microlensing magnifications we used a Bayesian method to derive
the size of the region emitting the continuum at 2558 angstroms. Analyses of
the Q 0957+561 R-band light curves show a slow but systematic increase in the
brightness of the B relative to the A component during the past ten years. The
relatively low strength of the magnitude differences between the images
indicates that the quasar has an unusually big optical accretion disk of
half-light radius $R_{1/2} = 17.6 \pm 6.1 \sqrt{M/0.3M_\odot}$ lt-days.
