the width is easy enough, but I always wondered about the top and bottom myself, but the other day one of the analysts said that they keep a running average of each hitter's last 100AB and the top and bottom lines are the average over those AB of the what the umps have actually called. If you follow a game on MLB gameday where the 'cartoon' has a fixed background, you can see the top of the strike zone box change a bit wrt the background on each hitter.