The size estimates provided are not instrumental readings- there's no information in the text that supports this interpretation. They are- as it makes sense and as clearly stated- estimates based on the best knowledge and understanding of the reporter at that particular moment. They change with the physical exploration of the base. So when the reporter says that the base is the size of the cosmos, his size estimate should match the size of the cosmos. It's pretty clear that here Ballard just chose a number that seemed both immense and a big enough jump from the previous estimate, but at a second look makes his character incoherent because he's contradicting what he just wrote.

> instrumental readings- there's no information in the text that supports this interpretation.

From the story: “A curious feature of the station is its powerful gravitational field, far stronger than would be suggested by its small mass. However, this probably represents a faulty reading by our instruments.”

And then the next paragraph is the estimated size.

> but at a second look makes his character incoherent because he's contradicting what he just wrote.

His character was incoherent from the first look, too.