We present eight monitoring seasons of the four brightest images of the gravitational lens SDSS J1004+4112 observed between 2003 December and 2010 October. Using measured time delays for the images A, B, and C and the model predicted time delay for image D we have removed the intrinsic quasar variability, finding microlensing events of about 0.5 and 0.7 mag of amplitude in the images C and D. From the statistics of microlensing amplitudes in images A, C, and D, we have inferred the half-light radius (at {λ }{rest}=2407 \mathringA ) for the accretion disk using two different methods, {R}1/2={8.7}-5.5+18.5\sqrt{M/0.3{M}ȯ } (histogram product) and {R}1/2={4.2}-2.2+3.2\sqrt{M/0.3{M}ȯ } lt-days ({χ }2). The results are in agreement within uncertainties with the size predicted from the black hole mass in SDSS J1004+4112 using thin disk theory.