Thanks for posting in the forums! I'm sorry to hear you are unable to get a full screen image when connecting to your Slingbox via iPhone 5.
I have seen that this sometime can be a form of High Definition Copyright Protection. Could you try disconnecting any HDMI cable connected to your set-top box, then test to see if the video snaps to full screen?
Also, if this does not work, you can also try changing the resolution of your set-top box. From 1080i to 1080p, or the other way around.
Hope this helps. Feel free to reply!
The Sling Moderation Team