For comparison, that's about 10% of the Moon's diameter, i.e. 0.1% of its volume. (The mass ratios are probably within that 0.1% ballpark, but can't tell for sure until we know more about its composition.)
Interestingly, the images came from the Dark Energy Survey [1], which for entirely different different reasons is running a very sensitive and high-resolution scan of the sky in visible and near-infrared. This just happened to show up in a frame where they were looking for distant galaxies and events, and the Minor Planets Center noticed the thing.