I found Firefish which is a better alternative to the standard Mastodon application. It’s compatible with Mastodon and has better features. It allows users to create web pages and the character limit is 3,000 instead of the 500 on Mastodon. It feels like a good limit for the type of site that it is. Plus this means that it can handle long posts from users on customized Mastodon instances.
The creator of Mastodon said that he isn’t going to add an option to set post length limits. I briefly tried to change it and wasn’t able to make it work.
It’s not configurable through the UI, but if you’re the admin of an instance you can change the character limit with some fairly simple source code tweaks.