Do you have any idea what the Aspect Ratio is of the picture from your device ? Sling have decided to support only 2 aspect rations in http://newwatch.slingbox.com and the Mobile apps and those are 4:3 and 16:9, so if your TV provider is using a slightly different aspect ratio then you are stuck with it.
Are you seeing the same on a PC ?
If you are not in the USA you could test this by going to http://watch.slingbox.com and testing the Zoom option there, as that supports 5 different Aspect ratios. One of those might work and tell you which Aspect Ration you need.