I've googled a resolution of height: 851px and width: 315px but it actually seems to be considerably larger than this.
The full image https://i.stack.imgur.com/lJAdn.jpg
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According to Facebook help it is indeed 851 × 315 (in pixels). The minimum size you can upload is 399 × 150 (but then it'll be stretched to fit).
Cover photos are 851 pixels wide and 315 pixels tall. If you upload an image that's smaller than these dimensions, it will be stretched to this larger size. The image you upload must be at least 399 pixels wide and 150 pixels tall.
To get the fastest load times for your Page, upload an sRGB JPG file that's 851 pixels wide, 315 pixels tall and less than 100 kilobytes. For images with your logo or text, you may get a higher quality result by using a PNG file.
Using MeasureIt, that appears to be accurate.
Your image seems to be enlarged