  We select 50 main-belt asteroids with a diameter between 20 and 400 km for
which we have (i) shape models derived by the lightcurve inversion method (LI)
and (ii) resolved observations of good quality collected with the Keck II
adaptive optics (AO) system in the near-infrared. We derive the size of these
asteroids by minimizing the difference between the contours from deconvolved AO
images and the projected silhouettes calculated from the shape model at the
time of the AO observations. We compute the volume-equivalent diameters for 48
of these asteroids. For 15 of them, we remove the ambiguity of the pole
orientation typical for shape models derived by the LI. We have found that our
equivalent diameters are smaller by 3%, 7%, and 2% compared with the effective
diameters derived from mid-IR photometric observations provided by IRAS, WISE
and AKARI. For 40 asteroids with previously determined mass estimates, we
compute their bulk densities and discuss the mass-density dependence with
respect to taxonomic types.
