Unfortunately Mastodon's "vanilla" version deliberately does not allow instance admins to easily adjust the character limit.
Some admins with tech skills can edit the code, and there are forks (Glitch Mastodon, Hometown Mastodon) which let admins easily adjust it, but unaltered vanilla Mastodon doesn't have this option 😞
I've done a list of good servers with larger character limits at https://fedi.garden/tag/larger-post-size/
