Looking at how city size relates to hole diameter.
If Phoenix, Arizona (pop. 1,469,471) had a ring it would be 800m in diameter – spot the hole:
And here is Las Vegas with a ring (harder to spot). With a population of just under 600,000 the hole would “only” be 500m across. Of course it would be a lot bigger were it to also cater for the estimated 40 million visitors per year.