In drupal 6 post settings is under the Content Management => Post Settings, Post Settings contains “Number of posts on main page” this is the maximum number of posts to display on single page, if more than that it will display as pagination.
And the next one is the “Length of trimmed posts” this is the maximum number of characters used in the trimmed version of a post that is teaser version of display.
Next one is Preview Post, it is required to preview post or optional.
In drupal 7 these are under Configuration=>System=>Site Information, you can find the number of posts setting under Front Page.
And the next setting is Length of trimmed posts in found under Structure=>Content Type=>’Select the content type‘=>Manage Display. You can find field named body and the format would be default change that to trimmed. By default trim length is 600, click on settings button and change the length.