1. Choose a theme designed to display full posts. Themes that automatically provide excerpts on the front page are listed here http://en.forums.wordpress.com/topic/theme-with-automatic-excerpts?replies=13#post-1396410
2. Create an excerpt - workaround to display full post.
This applies to any theme that provides automatic excerpts on the front page. As there is an active excerpt box below the post editor, you can control the length of the excerpt displayed and the exact excerpt that is displayed . This is the workaround you can use to override excerpt display on the front page.
You compose your whole post first and before you publish it you switch to the HTML (Text) editor, copy the code for what you wish to appear, paste it into the Excerpt box below the editor, and publish the post. http://en.support.wordpress.com/splitting-content/excerpts/#creating-excerpts
If you want to create your own excerpt for each post then that will have to be done on every post.