Change the number of characters in posts, bios, polls or others

Mastodon doesn't provide any setting to change:

All those settings are hardcoded in Mastodon, and changing those would require altering the code and recompiling Mastodon assets.

As mentioned in Mastodon version and code changes, no code changes are supported on infrastructure.

Everyone hosted on shares the same base code, and running the unchanged code is the best way to automate upgrades and maintenance. Sorry.

If Mastodon ever offers a setting to change any of those (like they do for other settings), will fully support it and provide a way to adjust it to your liking.