Cover images (only the actual proper cover) are displayed fullscreen, so I guess that would be 600x800 too
.
For images embedded anywhere else, it's a bit smaller, taking panels/margins into account. A quick screenshot and the ruler tool in any decent image processing tool should give you a precise answer
.
(Actually, I'm not even sure anymore if images are subjected to the white margins at all :/.)
I don't do much MOBI/ePub formatting, so take this with a grain of salt, I may be completely off
.