If the theme shows the full post, the author can control what shows in the list by using the <!--more--> tag. Only the content before that tag is shown, (so if it isn’t used the whole thing is shown).

If the theme shows an excerpt, the author can control what shows in the list by writing a manual excerpt for each post, which can include HTML tags and be whatever length desired. If there is no manual excerpt, the content is stripped of HTML tags and the default is the first 55 words to use as the excerpt.