Personally I don’t know why you would want to see a news post’s size in lines, but that’s the default in Thunderbird for showing the size of a news (NNTP) posts. The following is how to change it to show in data units e.g. kilobytes.
In the Tools menu, Select Options, Click the Advanced icon at the top, then click on the General tab, click the Config Editor button. Type
news.show_size_in_lines into the filter, set the value to False.
You can now close the about:config window, and you will now find that the posts in news group will be as a size not in lines.