I think the DX was 9.7". It was also very expensive. As I recall it was over twice as much as the normal Kindle of that day. I just googled it. It was $489 at introduction and the Kindle 3 was $139 at introduction.
At the time it was the only large e-ink ereader and now people are getting more used to the idea of a little larger screen.
I wouldn't be surprised if Amazon came out with a 7" or 8" Kindle at some point since there seems to be a market. I doubt they'll feel pressured to do that to compete with Kobo but they might see the Aura One's success, if it is successful, as an opportunity.
Of course Amazon is full of surprises so guessing what they might do is just that; guessing.
Barry