So small that a natural body of that size probably wouldn't be massive enough to hold a spherical shape. DS1 was a little smaller than the real asteroid 128 Nemesis, which isn't spherical. Maybe if it were made of something extremely dense, it would be, but you're not likely to find a natural spherical object that size.

Now that I think of it, this puts the "that's no moon" scene in perspective. Luke is a country bumpkin who just calls it a moon, but Obi-wan has an idea of its size (perhaps from glancing at the Falcon's scans, since size and distance is hard to judge by eye; or he's just a space wizard), and knows a natural object couldn't be that spherical.