Not sure this is the right place, otherwise be so kind to point me to a more appropriate one. Anyway, this might be useful for others, so here goes:
Twitch limits the visible chat message part of an IRC/TMI message to 512 bytes (510 without \r\n). In other words, the maximum length of a chat message is 510 characters in most cases.
However, the Twitch website does not enforce or communicate this limit in any way: you can type as many characters as you want in the chat input field and the message will show up in full on your side. All other chat users, however, will only ever see the first 510 bytes; in other words, your message might be truncated without you knowing it.
A fix would be easy, the input field could just use the maxlength attribute. Just be aware of this until such a fix is deployed.
EDIT 2019-07: The most recent changes deployed to the web chat actually implement a message warning the user when they reach the character limit and prevent input of any more characters.