There are many people who like to post a bit longer length but sometimes they get bored reading the extra long length posts and many people skip these posts. Many people feel bored while reading long posts but sometimes even long posts contain a lot of important information.
Well long post should not bored people if it has some useful and/or interesting information. If the long essays were the reason people do not read them, then we never would had the blogging websites and articles written on them to get the traffic, as all those articles are also long and take time to read.
Again, many people like to make short posts. In this case, even short posts may contain important information. Many campaign managers ask their posts to follow the campaign's requirements, which must include characters. There are many campaign managers who do not accept short length posts and in their case require posts between 150+/200+ characters. However, no matter if the post is big or small, if the post contains important information, then those posts will be healthy for the forum.
Very short post like half liner, one liner or even two lines are considered spam these days, especially if the writer of the post is wearing a signature. It is expected that since the writer got paid to post, he or she should put in some effort and make a reasonably long post (but that does not mean that he drags the post long meaninglessly only to extend it).