How can I increase the length of the excerpt of a particular blog post under a category. For now, one of my blog posts is showing only one line as an excerpt.
For excerpts you can either create manual filter (PHP shils required) or just add it manually for each post.
There should be Excerpt field under article in Post Edit screen. If there is no such field you can enable it via Screen Options located in the upper right corner of the thame Post Edit screen. There is no limits for Excerpt length when added manually.