> No, the main character isn't confused at all in his last message, he's very confident in saying that the station "is coeval with the cosmos, and constitutes the cosmos."

The main character is in the midst of a religious fervor. The station does not constitute the cosmos.

> "estimated diameter: 15k ly" feel like a writer's oversight.

Every time the instruments were checked, the instruments jumped in estimated diameter. Are you confused because the speaker got to the conclusion that the station is the universe before the instruments did? No doubt if there were more reports, it would be reported as being larger. Probably it wouldn't stop at the size of the universe, either.

In any event: neither the speaker, nor their instruments, were correct. Both are deeply confused about what the station is.

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.