An error occurred while saving the commentAndy Cook commented
After a lot of trial an error, I've figured out the exact length for a message to trigger the "Your message is too long. Please shorten it before sending" is 16,385 characters (at least on the desktop web app. I haven't tested mobile)
This seems to include any HTML that is wrapped around the text. If you copy and paste plain text in, it's actually longer because Teams wrapped it in the following HTML:
<div itemprop="copy-paste-block" class="copy-paste-block">
Even though "MESSAGE TEXT" is only 12 characters, Teams is actually registering the length as 78 characters.
The HTML formatting being taken into the character limit is why some "short" message might actually trigger this warning. It's very common for copied HTML to not be properly sanitized when pasted into a WYSIWYG editor like Teams has for messages.
If you're seeing this error on short, unformatted text, my suggestion would be to paste as plain text using (CTRLl/CMD + SHIFT + V). That should dramatically whittle down the length of your message.
Hope this helps! I was stumped why my shortish Adaptive Card for the integration I'm building was triggering this error, but it's because there was a lot of formatting in it causing it to go over the character limit even though the text content was short.