The website states that the minimum cover size is 1000 pixels on the longest side with 2500 recommended. (Huge)
The KDP ebook guide says that the minimum is 500 pixels and 800x600 recommended.
My own art is 800x600 and always appears ok.
The only thing it would affect is the image quality when viewed larger. Since I've started doing covers for other people, I want to get it right, in case there are issues with uploading on the respective Amazon sites. Does anyone know which it should be?
Mine are the older recommendation of roughly 800 x 600. However any new covers I do will be the larger size - and I may even re-do my older covers at the larger size.
Where possible, you should aim for 1000 x 2500. This is because of the newer generation of devices such as the Kindle Fire which have much higher resolution displays than older devices. Apple's new iPad also has a very high resolution 'retina' display which makes so called 'retina images' (in other words large images) look great. To put it simply, the larger your images, the better they look on newer devices.
2500 by 1000 will give you a very tall, slender cover which severely limits your ability to display the cover information effectively. Amazon’s sometimes-recommended 1.6 to 1 ratio gives a cover of 2500 by 1560, which is also a bit slender for me. I prefer a ratio where the width is 75% of the height. That makes a cover of 2500 by 1875.
You should try a couple of different layouts to see which you prefer, but, whichever you settle on, Amazon’s recent changes in Guidelines say to lean toward 2500 for the longest side.