Is the strike zone from the knees to the letters while the batter is in his batting stance or when he is standing up straight?

It is generally interpretted to be while the batter is in his batting stance. With that being said, the strike zone is different based on the umpires so you will not always have that hold true. In either case an umpire will not let you over exaggerate your stance to make you have an extremely small strike zone and gain a competitive advantage