Honestly, I don't really care that much. The project looks like fun, and I wish you the best with it, but I don't really post on microblogging platforms myself.

I just wanted to give a bit of feedback as someone who reads them - and point out that on almost every twitter/bluesky submission to HN they are using ways to work around the 280/300 char limitations.

To play devils advocate - 500 also seems a bit arbitrary ;) If I were making a platform like this myself, I would go for a power of 2 that makes sense for storage/buffering. Maybe 1KiB including all meta, etc. You could comfortably fit a post the size of your about page, and all associated meta in that - even when serialised as JSON or something. I'd go from there and work backwards to come up with a size that has a real technical purpose.

Make it a setting and let people decide for themselves.