I can't really answer your question, though I do think it's something of personal choice.
I prefer to scan my covers and reduce to a height of 850 pixels with the width being kept in aspect. As I don't really have 'that' good a scanner right now I scan at 300dpi and reduce accordingly.
I also like to scan my front covers to include the spines, which I think looks nice - it's something that some find very ugly!
Scanning and storing the images at this size I find I can view them without needing to scroll and that I also seem to notice much more detail in the artwork than looking at the actual box!
At the moment (and as I'm not capeable of web coding) the templates available for dvdPedia don't offer the option to have thumbnails from which you can access a full size cover item on a web export; therefore covers are only really postage stamp size, which is a shame.