Read Narwhal Microblog: How to remove max characters limit ? (wordpress.org)

The limit is for the post title. After you post, the plugin takes your post and creates a title using the first 40 characters of your post. This is for speed, so you don’t have to create a title. But, the content of your post does not have a character limit.

But, if you want to modify the title character limit it is easy to do.

Go to this plugin’s folder and open the narwhal-microblog.php file.

In this file you will see a line for this max character limit and you will see the number 40. You could just increase it to something like 100, 3500, or 999999. Depending on how long you are willing to let your titles get.