Facebook love change, and they had a little surprise for us all in March 2017! The Facebook event banner changed sizes very stealthily overnight. Not one site we found had the dimensions, so using a bit of web wizardry the WBS team worked out the perfect size for a Facebook event banner for 2017!
The displayed size is 500px wide and 262px high, however, if you create a pic that size combined with Facebook’s picture compression, it looks terrible.
So the optimal size for the absolute best resolution, we can advise you, is 2000 x 1024. We also recommend to save it as a .png NOT a .jpg (it just looks a little sharper).
Hope that helps!