depends on how you want to use 'em.
i just use them for displaying on my monitor, and stick to what
CDNOW uses, 170*170. That seems to work well for me.

If you want to print your covers, you could multiply those values by 10 or so.

Keep in mind that these huge files will make your database quite a bit larger, and thumbnail generation quite a bit slower.

cheers
-Bas