I have done several Kindle e-book covers and I use a custom preset of 1600x2500 pixels (16:25 aspect ratio). My author publishes with both Amazon and Smashwords, which distributes to other stores such as Apple and Barnes and Noble. The size meets the guidelines for both. For Amazon, check the Direct Publishing help site (kdp.amazon.com/help). I’ve found that you also need to make sure the cover will still look good and the title/author will be legible when it is shrunk to fit the web and e-reader devices.