I get this question a lot! In fact, probably more than any other question… so, to answer it once and for all.. here’s the answer (which incidentally has changed since the Kindle Fire came about): But the ideal cover size is a .jpg that is 800 pixels wide by 1066 pixels tall. This will result in a full size image on just about every eReader. This isn’t to say that other cover sizes won’t work… 600×800 is still very common, it just may be scaled to fit the screen (in some cases) and result in a slightly lower resolution. If you use a larger file, that’s fine as well – but doing so may result in a larger file size (& thus increase your delivery fee.)
*One caveat is Apple iBooks. They changed their cover requirements this year, and now require the cover image to be a minimum of 1400 pixels on the shortest side. So be sure to also get a ‘large’ version of your cover to meet Apple’s requirements if/when you decide to publish to the iBookstore.