The Laws of Prayer (82)

1. When one comes to take the three steps back at the end of the Amidah, one should lift up the left foot first to show that he finds it difficult to depart from standing before Hashem. (Mishne Berurah 123)

2. The length of these steps must be at the very least to put the heel next to the big toe, in the same way that the Priests did in the temple. Some say that if he took steps smaller than this size they are not called steps at all and that one may not be lenient even if the place where he is standing is narrow and cramped, whereas others are lenient when necessary. Ideally, one should not take steps which are bigger than this. (ibid)