"Based on its apparent brightness, dynamicist Bill Gray calculates that it would have a diameter of about 160 meters (525 feet) if it were a rock with a surface reflectivity of 10%."

It is, of course, almost certainly a rock. But as a pointless mental exercise, how large would it be if it were a shiny metal probe?

The observations are still coming in and things are very uncertain. One astronomer on twitter noted that if the most uncertain measurement were removed that the comet has a 0.9 eccentricity, and therefore likely not an interstellar comet.