  Recent proper motion studies of stars at the very center of the Galaxy
strongly suggest that Sagittarius (Sgr) A*, the compact nonthermal radio source
at the Galactic Center, is a 2.5 million solar mass black hole. By means of
near-simultaneous multi-wavelength Very Long Baseline Array measurements, we
determine for the first time the intrinsic size and shape of Sgr A* to be 72
Rsc by < 20 Rsc, with the major axis oriented essentially north-south, where
Rsc (= 7.5 x 10^{11} cm) is the Schwarzschild radius for a 2.5 million solar
mass black hole. Contrary to previous expectation that the intrinsic structure
of Sgr A* is observable only at wavelengths shorter than 1 mm, we can discern
the intrinsic source size at 7 mm because (1) the scattering size along the
minor axis is half that along the major axis, and (2) the near simultaneous
multi-wavelength mapping of Sgr A* with the same interferometer makes it
possible to extrapolate precisely the minor axis scattering angle at 7 mm. The
intrinsic size and shape place direct constraints on the various emission
models for Sgr A*. In particular, the advection dominated accretion flow model
may have to incorporate a radio jet in order to account for the structure of
Sgr A*.
