"How long is the Yangtze?"

Translation: 长江有多长？

[ktdilsiz]

how is this a question?

[DestinationVoid]

Question: 长江有多长

Statement: 长江很长

More info: https://resources.allsetlearning.com/chinese/grammar/Asking_about_degree_with_%22duo%22

Correct me if I' wrong, but I think you could omit 有 in this question.

[Dr_Jerry]

You could omit it. I've always heard a question of length posed this way: 长江是多长的?

[KX3.]

Yes, you can. The statement in response should be “长江有/一共/全长6300公里。”, the one you've given answers a different question, e.g. “长江长吗？”.