This OSIRISREx tweet says:
It's official! I'm in orbit around #asteroid Bennu -- now the smallest body ever orbited by a spacecraft. My snug path around the asteroid also sets a new record for the closest orbit of a planetary body by any spacecraft.
How close is OSIRIS REx orbiting Bennu? What is it's periBennu? And how does it compare to Dawn's periCeres? And if JAXA's Hayabusa 2 can be considered "in orbit" around Ryugu, to it's periRyugu?
How far inside Bennu's Hill sphere will this be?
(for more on that uncertainty, see Nature of Hayabusa-2's “Home Position” relative to Ryugu? Is it Ruygu's L1?
There are some numbers in AsteroidMission.org's NASA’s OSIRIS-REx Spacecraft Enters Close Orbit Around Bennu, Breaking Record and a nice animation in the video, but nothing really clear about the actual size of the orbit once the maneuvers are complete.