Just like Wikipedia, people follow changes. If there is vandalism, it often gets reverted within a few hours. If there's a mistake (relation broken for example) it can take a few hours longer to fix.
replyI 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.
replyIt 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.
replyThe 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.
The main method of prevention is blocks on users that make malicious edits. Obviously that doesn't prevent all malicious edits.
replyThe larger users of the data are doing some degree of validation of changes before showing them to users.