Mastodon.social has the default 500 character limit. There are a few which have changed that (because it's not a configurable setting). Most large servers on this list aren't blocking too many others and aren't blocked by others https://fedidb.org/software/mastodon (Maybe don't choose mstdn.jp)
I host https://mastodon.world myself but there are many others like it. For me, choices could be based on:
- What's the server topic (if any)
- Is the server run by a single person, or a team? (For continuity reasons, but this info is mostly hard to find)
- Can I agree with the server rules?
Also, you could consider other platforms which are similar to Mastodon, and can communicate with Mastodon, but might have some features you prefer which Mastodon hasn't't (like quote posts, bigger character limit etc). For my personal account, I use Sharkey (https://sharkey.world is mine too), but you can also consider Pleroma, Friendica and many many others.
Sorry if I made the choice more complicated... and let me know if you have questions.