To calibrate the size of Control Center's interface so that it properly fills your screen, navigate to Settings > General > Calibrate Screen:
Use the up and down buttons on the directional pad on your Control Center remote to adjust the size of the interface. Set the size so that the arrows on this screen just touch the edges of your display:
If you find that all 4 arrows don't touch the edge (image is slightly off-center), you may need to use your TV remote to access your television's display settings. You should then be able to manually center or switch to an automatic setting.