Have you also read the theme descition here?
To get started with the custom Showcase Page Template you need to create a page for it. Choose the Showcase Template under page templates in any page, save it, and assign it as the front page in Settings → Reading. This will change your site from the standard Twenty Eleven blog look to Twenty Eleven’s fancy showcase. You still have you regular blog list at hand for several views, like categories, searches, and tags and you can specify another page to work as your blog if you want to.
To add posts to the Featured Post section just mark some posts as sticky. That’s all there is to it.
To have a big featured image you need to use an image that is at least as big as the header image (1000px x 288px). Also, make sure you have a set size for Large image under Settings → Media that allows for such sizes. ... http://theme.wordpress.com/themes/twentyeleven/