In Post Timeline some options are available to control the Post Description length.
These options are:-
1.) Post Timeline >> Content >> General Settings >> Post Description :- Summary or Full
2.) If you choose Summary then in Content Lenght option you can limit the number of words to display in Post Description. Post Timeline >> Content >> General Settings >> Content Length
3.) Read More link is also there along with post description Summary. And you can add your custom text in place of Read More (default). Post Timeline >> Content >> Advanced Settings >> Read More Button text
4.) If you don’t want to display Read More link with Story Description then you can hide it from Post Timeline >> Content >> Advanced Settings >> Read More Button
5.) If you choose “Full” in “Post Description” option then Text Formatting in Post Description will work. Text formatting will not work with Post Description = Summary