  We combine high-resolution images in four optical/infra-red bands, obtained
with the laser guide star adaptive optics system on the Keck Telescope and with
the Hubble Space Telescope, to study the gravitational lens system
SDSSJ0737+3216 (lens redshift 0.3223, source redshift 0.5812). We show that
(under favorable observing conditions) ground-based images are comparable to
those obtained with HST in terms of precision in the determination of the
parameters of both the lens mass distribution and the background source. We
also quantify the systematic errors associated with both the incomplete
knowledge of the PSF, and the uncertain process of lens galaxy light removal,
and find that similar accuracy can be achieved with Keck LGSAO as with HST. We
then exploit this well-calibrated combination of optical and gravitational
telescopes to perform a multi-wavelength study of the source galaxy at 0.01"
effective resolution. We find the Sersic index to be indicative of a disk-like
object, but the measured half-light radius (0.59+-0.007+-0.1 kpc) and stellar
mass (2.0+-1.0+-0.8e9Msun) place it more than three sigma away from the local
disk size-mass relation. The SDSSJ0737+3216 source has the characteristics of
the most compact faint blue galaxies studied, and has comparable size and mass
to dwarf early-type galaxies in the local universe. With the aid of
gravitational telescopes to measure individual objects' brightness profiles to
10% accuracy, the study of the high-redshift size-mass relation may be extended
by an order of magnitude or more beyond existing surveys at the low-mass end,
thus providing a new observational test of galaxy formation models.
