I'm curious about this. I always make my covers 1607 x 2500. And I know I sized that up from back when my books were 1200-ish pixels on the longest side. But I can't remember where I got the initial numbers from.
Amazon likes 2500 on the longest side, and roughly tall and thin. What you do with that is up to you. We recently redid the kindle covers making them wider and are happy with the effect. They now have the same 6x9 ratio as you would see on a print cover. (Not sure what the ratio is on the "recommended" amazon cover, it's less than 6 though.)
Generally checking with each individual outlet is how you build the list you need. I currently work with 6x9 kindle (2500 on the long side) and 600x800 for epub.