  We use gravitational microlensing of the four images of the z=0.658 quasar
RXJ1131-1231 to measure the sizes of the optical and X-ray emission regions of
the quasar. The (face-on) scale length of the optical disk at rest frame 400 nm
is 1.3 10^15cm, while the half-light radius of the rest frame 0.3-17 keV X-ray
emission is 2.3 10^14cm. The formal uncertainties are factors of 1.6 and 2.0,
respectively. With the exception of the lower limit on the X-ray size, the
results are very stable against any changes in the priors used in the analysis.
Based on the Hbeta line-width, we estimate that the black hole mass is ~10^8
Msun, which corresponds to a gravitational radius of r_g~2 10^13 cm. Thus, the
X-ray emission is emerging on scales of ~10r_g and the 400 nm emission on
scales of ~70 r_g. A standard thin disk of this size should be significantly
brighter than observed. Possible solutions are to have a flatter temperature
profile or to scatter a large fraction of the optical flux on larger scales
after it is emitted. While our calculations were not optimized to constrain the
dark matter fraction in the lens galaxy, dark matter dominated models are
favored. With well-sampled optical and X-ray light curves over a broad range of
frequencies there will be no difficulty in extending our analysis to completely
map the structure of the accretion disk as a function of wavelength.
