Several sites and forum or general site engines use or at least provide the possibility of limitation in the length of posted comments, posts and/or replies. Stack Exchange engine, for example, has a quite strict limit in case of comments.
What can help determining the ideal length? Too low limit might cause mild frustration in the user.
I have found in all of my designs the limitation on how big a comment can be is based either by the business not wanting to read long comments, or by some Tech Arch / Data Arch claiming that it will take too long to fetch or will take up too much space in the data base.
From an ideal perspective there is no reason to limit what is captured, just put in a visual break on the content around the 300 - 500 character mark with a "more..." type prompt at the end. This allows the reader to get some context and choose to read the rest while exposing more of the other information on the screen to the user.
I think length restrictions can be informed by the behaviours the site wants to encourage and/or discourage. A large character limit can encourage users to engage in discussion or to write detailed and well researched posts. While, a small character limit would set an expectation for directness and brevity in posts.
This can work well in the aggregate, but is not fool proof. StackExchange wants to ensure that comments are not used to post answers. But the 600 character restriction cannot prevent contributors from writing short answers as comments. Sometimes the limitation is just technological. Twitter's 140 character limit is a holdover from its origin as an SMS service (SMS has a 165 character limit).
I think the frustration of hitting the limit is a reminder to the user that their current expectations don't align with the sanctioned behaviours for the site. If the limitation is unusual then the site probably needs to have an explanation near at hand. Again, StackExchange does a good job of this the placeholder text for comments tells the user what should and shouldn't be there.
