If you're wondering why some people have much larger character limits, the explanation will be one of these:

-They are on a Mastodon server where the admin has tweaked the code to allow higher limits (this is a bit tricky, vanilla Mastodon doesn't officially support this)

-They are on a fork of Mastodon such as Glitch or Hometown, which support adjustable character limits

-They are on a non-Mastodon Fediverse server such as Friendica or Calckey or WordPress which support much higher limits