Actually the KDP guidelines say:
Requirements for the size of your cover art have an ideal height/width ration of 1.6, this means:
• A minimum of 625 pixels on the shortest side and 1000 pixels on the longest side
• For best quality, your image would be 1563 pixels on the shortest side and 2500 pixels on the longest side
I just looked into an Amazon sample that had a cover 860 x 1226 px. The ratio is 1.426. So anything goes?
As for GraciousMe, who said in another thread she doesn't publish at Amazon, a cover with a size of 800 x 1000 px would be ok, though it has not the ideal
ratio of 1.6 recommended by Amazon.
For KDP I found some useful information on:
"A last note on this: these images are the display images Amazon uses to sell books on their site, NOT the cover image included in the book. In most cases, it is advantageous to include in the book file an image on the lower end of the spectrum, since readers will rarely actually see it there, and higher quality images result in a larger files size for the ebook. Since indie writers getting the 70% royalty from Amazon are billed a small amount for file downloads, the bigger the file, the less you earn per sale. Those pennies can add up in the long run, so best practice is to include in the book file a smaller file size of image than the one you’re using for display."