I once encountered a joke building that was so large, I remember in the magnitude of 500km2, that you couldn’t edit it in the web editor. You had to download a desktop application to delete it.

It was in a place of the world that is unlikely to be popular within the OSM community.

Fortunately it's fairly easy to query the database for that kind of thing and a few editors periodically go through the most implausibly-sized common features, so anything too egregious will get noticed eventually.

The last time I saw a collection of "biggest objects" for various tags it was mostly things that were just subtly mistagged (e.g., an entire forest being tagged as a single tree) rather than vandalism.