Like most I’ll do a Google search when I’m making a new Facebook cover photo to find the dimensions. Most sites will tell you what Facebook themselves tell you, it’s 851px x 315px and these have been the dimensions I have used when creating cover photo’s for our client pages.
However, I recently uploaded a personal photo to my own profile and was surprised to see that within the mobile view on an iPhone app it shows more height if it’s available. So my recommendation now would be to add an image of at least 851 pixels wide but include more height to cater for the new mobile view (see grab below) …
