The Midrash, as quoted by the Bet Yosef, learns from the verse “Kal Atzmotai Tomarna Hashem Mi Kamocha” (lit. All my bones shall say: ‘Hashem, who is like You!’) (Tehillim 35:10) that every part of one’s body should be used to praise Hashem, including one’s left foot when one finishes the Amida. Based on the Midrash, it seems that it is proper to take the three steps back starting with one’s left foot. The Bet Yosef and the Mishna Berura (O.H. 123:14) explain that normally when one approaches prayer or when one goes on one’s way it is done with the right foot first, but at the end of the Amida one is simply going back and it should be done with the left foot.
The Shulhan Aruch (O.H. 123:3) indeed rules that one should step back with the left foot first and that the minimum length of each step should be so that the toes of one foot line up to the heel of the other. The Rama (ibid.) says that ideally the steps should not be greater than this length, while the Magen Avraham says that they should not be shorter than this distance. As such, even if there is someone behind one who is praying, the Magen Avraham says that one needs to go around the person in order to make the requisite steps. The Bach says that in such a case one can take smaller steps in such a case. The Siddur Bet Oved points out that the Shulhan Aruch gave a minimum distance but that one may go beyond this distance. The Moroccan custom is to follow the Shulhan Aruch but in time of need, such as an obstacle or another person, one can adjust the steps accordingly. It should be noted that one should not knowingly begin one’s prayer if there is someone directly behind.
Summary: When taking three steps back after the Amida, one begins with the left foot. One should step back at least so that one’s toes line up with the heel of the other foot, or greater. If necessary one can step back less than this distance.