I understand that you want to know how to adjust the settings in order to fill the screen on your Android Mobile device.
As a matter of fact, this issue is related to the resolution that you are using on your cable box or DVR. With this in mind, I recommend you to check and play around with cable box resolution settings in order to get the right resolution that will allow you to fill the screen on your Android Mobile device.
Keep in mind that you must use Component cables (only the Slingbox SOLO and PRO-HD models have component inputs and support HD sources) if you want to get the full screen video on your Mobile device. The following link will provide you with all the information about this specific situation.
Can't view full-screen video with SlingPlayer Mobile for Android