If you have your cover XML in your ePub setup correctly, it won't matter the size of the cover image as it will then resize to fit by shrinking or enlarging as needed. What I do is if the cover is larger then 1024 lines, I reduce it to 1024. That fits nicely in all cases I've found. If the cover image is smaller, I just leave it be.
If you do find your cover image to be 510x680, it's aspect ratio is off and you'll need to find another cover image that's aspect ratio correct..