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 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 lightcurve inversion method. 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.