Wouldn't it basically just be the size you need to know? The content is entirely up to you since it's a custom game cover. Just apply all your previous graphic designing knowledge to this. If you intend to print it out so that it fits then you just need the dimensions. The dimensions of the picture in the link you gave us was 510 x 650. If that one had the correct sizes then I'd suggest going by that.
This is unless I'm not understanding your question.