As KingSparta said, it's really just personal preference.
As for going off the screen, mine do that with the normal trackinfo's (see below for sizes)... so I just editted the html page (that's what the trackinfo pages are, just html) to restrict the image to 250x250 so it looks nice on the page (lines up with the top :))... if you need more help doing this or just want my trackinfo, let me know (I also added lyrics to the first page so I don't need to keep changing pages to view the lyrics for each song, and made the artist/song name big so I can read them from my couch... modified from the DJ trackinfo).
I scan mine in myself at around 944x944 :) (varies a little per cover, I haven't equalised them all yet)... I've got a couple at around 1400x1400, but since that's bigger than my full screen res (at least vertically) it has to be shrunk down when viewing the image full screen (and I don't -really- need them that big :) (though I scan my DVD covers at 3200x2100 so I can zoom in and read that really tiny text :)).