I can only speak for the game requirement and common sense. You generally don't need battle sprites larger than 200x200. Portraits are even less than that, maybe 120x120. Most 'content' images are the file-size I mentioned earlier. Upsizing the files makes NO sense as the game will do so anyway and you are not likely to get better quality.
A lot of thought was out by Dark into making the packs which we have now, so you can always take a look at those and act accordingly!