Upon reading your post, I understand that you are having issues with the aspect ratio while viewing.
I have included a few articles that may be of assistance to you.
Here are some Knowledge Based Articles for you to take a look at:
- Slingbox.com - Why doesn't the video fill the whole screen?
- Slingbox.com - What’s going on when I change video zoom modes?
- Slingbox.com - The aspect ratio of the video is wrong
- Slingbox.com - Working with viewing formats
Take the Slingbox out of the equation and connect your Cable box directly into your television set. You will want to ensure that the viewing is correct and if neccessary make changes to the settings on the cable box. If you need assistance with that, contact your cable service provider.
As a Last Resort:
I hope that the articles were helpful