This may be a setting on the cable box that you may need to change. But in the meantime, you could try adjusting the aspect and zoom settings on the SlingPlayer.
This article below will walk you through all the functions on SlingPlayer for Web:
Sling Media Moderation Team
So I finally figured it out all by accident. I changed out my reciever/amp with a different model and now I have full screen. Apparently although I'm running the stream from my cable box to the sling with the component cables. The old amp(5 months old) affected the HDMI output from the cablebox that it wouldnt let it broadcast the proper hd stream. Yay full screen.......