@nprofile1q... infosec.exchange has a 11,000 character limit, which is big enough that I've never hit the limit.
Smaller limits exist because of a theory that it encourages concise posts. There is zero evidence that this actually works. In practice, people write long things and split them across many posts, and write 3/11 or whatever at the end. This ends up being much worse both for usability and performance: sending a single 4,000-character post across ActivityPub requires almost the same amount of data transfer as a 280-character one. But sending ten 280-character posts takes a lot more.
I wish Mastodon would make the defaults sensible instead of requiring instances to patch it.