  We exploit the strong lensing effect to explore the properties of
intrinsically faint and compact galaxies at intermediate redshift, at the
highest possible resolution at optical wavelengths. Our sample consists of 46
strongly-lensed emission line galaxies discovered by the Sloan Lens ACS (SLACS)
Survey. The galaxies have been imaged at high resolution with HST in three
bands (V_HST, I_814 and H_160), allowing us to infer their size, luminosity,
and stellar mass using stellar population synthesis models. Lens modeling is
performed using a new fast and robust code, klens, which we test extensively on
real and synthetic non-lensed galaxies, and also on simulated galaxies
multiply-imaged by SLACS- like galaxy-scale lenses. Our tests show that our
measurements of galaxy size, flux, and Sersic index are robust and accurate,
even for objects intrinsically smaller than the HST point spread function. The
median magnification is 8.8, with a long tail that extends to magnifications
above 40. Modeling the SLACS sources reveals a population of galaxies with
colors and Sersic indices (median n ~ 1) consistent with the objects detected
in the field with HST in the GEMS survey, but that are (typically) ~ 2
magnitudes fainter and ~ 5 times smaller in apparent size. The closest analog
are ultracompact emission line galaxies identified by HST grism surveys. The
lowest mass galaxies in our sample are comparable to the brightest Milky Way
satellites in stellar mass (10^7 solar masses) and have well-determined half
light radii of 0."05 (~0.3 kpc).
