If you want to setup facebook cover then you have to know about what is the minimum size required for facebook cover photos.
Facebook official help center answer on the size of cover photo that your cover has to be at least 720 pixels wide .Here is details:
What if the image I want to use as my cover is too small?
What Cover Image size to use?
Its is a good idea to upload a high resolution image with exact size of 850px wide and 315px height which fill fit perfectly.
What is the minimum width of the Cover Photo?
Officially Facebook FAQ says your cover image must be at least 720 pixels wide. Smaller images will get upload errors. I tried to upload a smaller image and got a message to upload cover photo at least 399px wide. I was able to upload images 399px wide and above (which were zoomed out and could not be repositioned). Any image height is accepted it seems.
What if you use cover photo of any other size?
Firstly Facebook Cover photos below allowed size will not be accepted for Cover Photo upload. If you upload any other size, Facebook will be forced to resize / virtually crop the image, which will make it distored and pixelated. Upload the exact size.
Can you crop / edit the Cover Photo?
Facebook will only allow you to reposition the Cover image if you upload a larger image . For most large images, this will work more like moving the image ‘up and down’ till you get all the area of your interest in the fixed size of 850×315. Then you can save your Facebook Cover.
Can other people save Cover Photos?
This image is visible for public use and can be saved. What is important is that this is like a virtual crop, so if people decide to right click and save your Facebook cover, they will save the full uncropped image. e.g. if you upload a big full size photo of yourself, and then decide to make the head as you cover photo, people saving the image can see the full image. So its a good idea to upload the exact size.
Remember all cover images are public, so choose wisely as all will be able to see them.