Copy and paste is your friend here. If there's 20+ huge PRs, just paste "This PR is far too large to review, please break it down and submit smaller PRs and engage with us ahead of time to understand how to solve this problem."

Comment & Close PR, only engage in discussions on tickets or smaller, understandable PRs.

As other have said: if someone drive-by opens a huge PR, it's as likely to be malware as a beneficial implementation.

You can read one or two and decide if it's worth going through. Otherwise you can just reject them.