You might not be able to get a "full screen" picture. However you might be able to fill the screen better. I personally found the fullest picture for HD can be achieved throught he component connectors and the aspect set to letterbox. However, for SD then the standard aspect with either the S-Video or Composite connectors.
Take a look at this post (https://community.sling.com/message/12137#12137). It will help you identify the difference with the HD and SD connectors, the effects with the different aspect zooms, and the difference with the HD and SD channels.