We present a measurement of the accretion disk size of the quadruple lensed quasar HE 0435–1223 from well-sampled 13-year 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/0.3 {M}ȯ } lt-days (histogram product) and {R}1/2={7.7}-1.2+7.0\sqrt{M/0.3 {M}ȯ } lt-days (χ 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 narrowband photometry of HE 0435–1223.