Mastodon is meant to be for microblogging (a la Twitter), but there is other software that accesses the same federated network (called the “Fediverse”) that focuses on other things, like long blogs (a la Medium), pictures (a la Instagram), etc…

(There are forks of Mastodon that increase the message length but on vanilla Mastodon it’s hardcoded to 500. But the fediverse itself doesn’t have a limitation like that, it’s a Mastodon thing)

The default is 500 chars iirc, but yeah. Entirely configurable (and many modify it in practice), and most servers will pull in much larger ones than they allow to be published, so you can get a novel in your feed if you follow people who do that.