From Joel Friedlander:
I’ve been getting a lot of questions recently about e-book covers and, specifically the size and proportion requirements for submitting the cover image of your e-book when you upload it to a retailer or a distributor.
Here’s a selection of requirements that should help. Keep in mind this information is accurate now but could change at any time, so check when you’re ready to upload.
Kindle Direct Publishing
From Kindle Direct Publishing help system:
. . . .
Requirements for the size of your cover art:
At least 500 pixels horizontally and 800 pixels vertically
Ideal height/width ratio of 1.6
Maximum of 2000 pixels on the longest side is preferred
Save at 72 dots per inch (dpi) for optimal viewing on the web
. . . .
Barnes & Noble Pubit!
From Barnes & Noble Pubit! Help system:
Your cover image must be a .JPG or .JPEG file with a file size between 5KB and 2MB. The sides must be between 750 pixels and 2000 pixels in length.
Link to the rest at The Book Designer
There’s nothing earth-shaking here, you would have to hunt all over the place to find this information, so, as usual, Joel has been very helpful.