You might have noticed that some accounts on here have posts much longer than 500 characters. If you're wondering how this is possible, find out in this guide:
➡️ https://fedi.tips/why-do-some-people-on-mastodon-and-the-fediverse-have-bigger-character-limits
TL;DR - You need to be on a server that allows larger post sizes, either running a modified version of Mastodon or a totally different kind of Fediverse server.
You can browse some well-run servers with larger post sizes in the "larger post size" section of fedi.garden: