It depends on the maximum width of your page content area. In most cases, the full width is from 900px up to 1200px. So the width should be up to 1200px for full width pages. If you have a sidebar, then the 900px is more than enough.

• Min width 700px (page with sidebar), 1200px (full width page).
• Min height 200px (page with sidebar), 400px (full width page).

The approximate aspect ratio of the cover image is 3:1

BTW, the CSS is will fit all size of images to the category background area, so you can attach any size of image as well.

Would be a big time saver, and not resizing 3-4 times.

Appreciate and px

thanks