[D_Idaho]

This story and the accompanying “artist’s concept” both appear to have been pulled out of someone’s...
Oh, forget it.

[itsahoot]

This story and the accompanying “artist’s concept” both appear to have been pulled out of someone’s...

Hey now, that is a picture from one of our satellites.

[Alas Babylon!]

Not at all. There is science that can be used to get a good idea of its physical dimensions:

The Physics Behind the Strange Interstellar Asteroid 'Oumuamua'

But if you can't see it, how can you describe it? The answer to this (as in many situations in science) is to use indirect observations. The one thing that can be measured is the brightness of the object. Because this rock is also spinning, the light it reflects from the sun changes over time. By looking at the ratio of the brightest to weakest observations, you can get an estimate of largest to smallest size. If you estimate the albedo (a measure of reflectance), you can also estimate the total size. Boom. There you have it—a cigar-shaped asteroid.