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It's slightly tricky as there's no official way of altering the character limit on standard Mastodon, admins have to manually edit several files and then possibly change them again for every update.
For whatever reason the Mastodon developers haven't included this in the options that admins can set within the official interface. Hopefully that changes in the future.
But yes, some admins are great and will make this change anyway! 
p.s. I've recently added many more servers to the "longer posts" list, so if you've checked it previously it might be worth checking it again.
Would you like to create posts longer than 500 characters on Mastodon?
You can have longer posts by moving your account to a server that offers longer posts. I've done a list of well-run reliable servers with larger post sizes at:
https://fedi.garden/tag/larger-post-size
If you're not sure how to move your account to a different server, there's a step-by-step guide at https://fedi.tips/transferring-your-mastodon-account-to-another-server
In case you missed it, the Fedi.Garden website is now designed to make it as easy as possible to choose a reliable server on Mastodon and the wider Fediverse:
The new design takes people to a selection that they feel comfortable with:
- If you find choice overwhelming, it shows you one good general server (which is regularly rotated to spread the growth)
- If you enjoy choice, it shows the full directory of good servers sorted into categories
Tube Free is a general video hosting server intended to help people join PeerTube. Backed by BT Free, a non-profit dedicated to digital privacy rights, advocacy and consulting.
You can find out more at https://tubefree.org/about/instance/home
You can contact the people who run Tube Free at https://tubefree.org/about/contact