PRs are as long as they need to be and as short as they need to be. This idea that any problem can be decomposed into 100-200 line changes is ridiculous. That's not realistic in many cases (esp. for refactoring work, etc.)
There are certainly exceptions. And if a problem cannot be decomposed, then you have a "stop the world" formal code review with a team-wide multi-hour dive into the code.
In my world, though, we use the refactoring tools such as feature flags and incremental improvements. We can use stacked pull requests. It requires training and discipline but it's absolutely doable for 99% of tasks.
