Searching online for 'size of Facebook cover photo' returns '820 pixels wide by 312 pixels tall', but this does not account for how Facebook renders the image. When set to this configuration, Facebook shrinks and crops the photo to an actual size of 851px x 315px. Therefore if you want to avoid shrinking and cropping that will reduce the image quality and make it blurry, then use 851px x 315px (width/height). Of course, your image should be a high-quality jpg/jpeg or png, so at these relatively small dimensions also best to avoid any reduction in quality with image optimisation.
Recommended Facebook Cover Photo Size to avoid blurring
Use the following Facebook Cover Photo image dimensions to avoid blurring: -
851px x 315px (image width/height)
Sometimes it's better to focus on quality rather than using a one size fits all approach.