I'm sorry also.A small comment in the text box would help participants know any policy about the desirable length and amount of daily comments. This already happens if a title is too long.
There is a length restriction on the body of a post as well. The official policies for Duolingo can be found in the Guidelines and Terms of Service There are certain norms within the community that are not official but are considered a standard among people who share the forum. You can find those on the wiki HERE.
I hope this helps! :)