By stellar occultations. Because angular speed of movement of asteroid on the sky is precisely known, observing a few occultations allows for precise measurement of it's diameter. That one is easy. But determining mass is not at all easy (and no you don't do it by observing it's orbit, it will be the same regardless of mass - you do it by observing orbits of all other things - especially those that happened to pass close to it). For heavy objects it's not that hard but for such a small thing, exceedingly hard.