The limit is “4096” characters, and any amount below that produces an error message (but if you’re not careful you might miss the red banner saying your message failed to send). I could not reproduce a message getting lost in my own testing.
In addition, these forums are for integrating Twitch with third party applications (not for technical support). To report a Twitch bug, use the bug reporting tool: http://www.twitch.tv/bug_reports/new