I don't know what the largest image size is that won't be resized on the Kindle (in a MOBI/AZW e-book). However, so far as I can tell, images are always resized to allow the usual header and footer and the usual margins. I think the aspect ratio is also maintained. For an example, see: Manga/pdf in the Cybook? Help, please! (post #20)
. Note that in the same thread, HarryT recommends 400x600 as the "standard" size for cover images (these are not handled quite the same as embedded images, but this size sounds safe).