On tex.stackexchange, I had to break a chat message into pieces because it was too long. Comment fields on the list itself say how many characters are left, but chat messages don't, and I can't find information on their size. (I probably just don't know where to look.)
Chat messages do seem to be smaller than comments, and a friendly co-member of meta.tex.stackexchange confirmed this by actually counting what he typed in (about 600 for comments vs. 500 for a chat message). Since the reason for my being in the chat room was in answer to a request to divert "extended comments" to a chat, I find that request rather impertinent.
A clear indication of the permissible size of a message, or a count-down a la comment, or even both, would be very welcome. It would also be welcome to have a message size that is at least no smaller than that of a comment in the main list.