on the epub part of the forum there is a thread (I believe the CSS tip thread) where there is more about covers. In there the complete code is given for a cover page which will size the cover to be 800x600, but maintaining ratio. It is in a svg wrapper. I will try to look it up.
Found it:
link
I always use this as my coverpage, with my own small adjustments. But, it will work as said.