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Mastodon comes with a default post size of 500 characters. This is not set in stone, and can be increased fairly easily (last tested on Mastodon 4.0.2):

Edit live/app/javascript/mastodon/features/compose/components/compose_form.js, search for the number 500 and replace it (should pop up twice)

Edit live/app/validators/status_length_validator.rb, search for the number 500 again and replace it (should show up once)

Edit live/app/serializers/rest/instance_serializer.rb, search for the block

attributes :domain, :title, :version, :source_url, :description,
           :usage, :thumbnail, :languages, :configuration,

and add , :max_post_chars behind :registrations, so it becomes

attributes :domain, :title, :version, :source_url, :description,
           :usage, :thumbnail, :languages, :configuration,
           :registrations, :max_post_chars

At the bottom of that same file, before the keyword private, add the following block (replacing YOUR_POST_LENGTH with the number of chars you want):

def max_post_chars

Now recompile Mastodon (as the Mastodon user):

$ cd live
$ RAILS_ENV=production bundle exec rails assets:precompile

Restart Mastodon (as root, or via sudo):

# systemctl restart mastodon-sidekiq.service mastodon-streaming.service mastodon-web.service

Now your post length will be whatever you chose ;)