I feel like I blog about this every other week.  It is ever changing.  Different sizes display differently on phones, tablets and laptops.  Here are some basic templates for headers and images.

Page Header Images
The number are confusing!

• 851px width x 315px height: recommended size for desktop
• 820px width x 312px height: display on desktop
• 640px width x 360px height: display on smartphones
• 400px width x 150px height: minimum cover image size

Bottom line: 851 by 315 displays well everywhere.  If you use an app like Canva, the templates are already optimized for you.

Many screens have enhanced resolution.  If you want your graphics to look amazing you can go with a larger template.  1,702 px width x 720 px

Event & Group Headers
Thankfully the same size can be used for both 1640 by 856.  In groups, there will be a non-visible area at the top and bottom.