What the Cisco review study found
SmartBear’s study of code review at Cisco Systems, covering about 2,500 reviews, found reviewers most effective below about 200 lines. Defect density fell off after that, and the authors treated 400 lines as the upper bound for a single review. The study assumes defect density is roughly constant per line, which is not true of every change. It is still the clearest public evidence that an unbounded pull request gets a weaker review.
What Warestack checks
The catalog rule Require concise pull requests flags a pull request whose changed-line count is over the maximum you set. Lock files can be excluded. The count comes from the pull request. Warestack does not open the diff and does not choose the limit for you. The check reports until you require it in branch protection.
Set the limit yourself
The catalog rule is off until you enable it, and it does not block a merge until you require the check.