Well, first of all before you begin working you need to set up your composition with correct frame size. For that go to File>New and then write the size you want in Width and Height. If you want widescreen 848x480 should be fine, just like Soup suggested. And when you're rendering the video out push Custom button in rendering window and make sure that width and height are the same as in your composition. Hope this helps. If you have any other questions, feel free to ask
