My iTunes is about 157mb, and you can find details on the size of iPods here: http://www.apple.com/ipod/compare-ipod-models/

(Perhaps you should give us some more information on what, exactly, you want! Or use a ruler to measure some CD covers you already own!)

according to apple:

A 300x300-pixel JPG is the recommended size and format.

but any larger format square image should do...

thanks people, I am sure I will figure out now...

You should be more concerned with what the distributor distributing to iTunes is looking for, this can be found on their site, its many times larger than a 300x300 pixel image for the most part.

I can only speek for myself when I say that I prefer covers which are at least 500x500px, considering the growing number of large high resolution screens.