  We present a measurement of the accretion disk size of the quadruple lensed
quasar HE 0435-1223 from well-sampled 13-yr COSMOGRAIL optical light curves.
Using accurate time delays for the images A, B, C, and D, we modeled and
removed the intrinsic quasar variability, and found microlensing events of
amplitude up to 0.6, 0.4, and 0.5 mag in the images A, C and D respectively.
From the statistics of microlensing magnifications in these images we use
Bayesian methods to estimate the size of the quasar accretion disk. We have
inferred the half-light radius for the accretion disk using two different
methods, $R_{1/2} = 7.6_{-1.1}^{+12.0} \sqrt{M/0.3M_{\odot}}$ light-days
(histogram product) and $R_{1/2} = 7.7_{-1.2}^{+7.0} \sqrt{M/0.3M_{\odot}}$
light-days ($\chi^2$ criterion). The results are self-consistent and in good
agreement with the continuum size predicted by single-epoch spectroscopy and
previous studies making use of narrow-band photometry of HE 0435-1223.
