The recommended minimum size for the banner picture on a user profile page or groups page is 1600 x 300 pixels.
The recommended minimum size for a profile picture on a user profile page or groups page is 200 x 200 pixels.
The background image in the profile page expands as wide as the screen size of the user viewport. It is advised that you upload a wide image that supports the big screen. The height of the image is fixed at 300 pixels.
The background will automatically cover the whole space and centralizes itself.