I never use anything within a book larger than 800x600 for a few reasons. It suits most ereaders nicely and any reader should be able to scale it to fit full screen even if their resolution is 1024x768 or larger for tablets with the correct code. Even if some device is capable of displaying a larger image, the cover isn't the main point, the text of the book is.
The size of the image file itself won't be as large. Nothing annoys me more than an epub (which are usually between 250-600kb in total size) that is over 2MB in size because the cover is over 1MB. Even if you do go with a larger image, just please remember to compress it with a decent graphic program. If done correctly, you won't even see the difference but will reduce the file size to as little as 12% of what it originally was, dropping it down from over 1MB to 100kb.
Not all readers have slots for SD cards, so we don't have unlimited space.
Last edited by Ripplinger; 03-14-2013 at 12:22 PM.