This page gives a rundown of the 6 orbital elements and how to think about them in STK.  While the actual Classical Elements have Semi-Major Axis and Eccentricity, the linked videos adjust these from the Classical to focus on Apogee & Perigee which are often more functional for our Space Missions.  Apogee & Perigee together adjust the same effects as Semi-Major Axis (size) and Eccentricity (shape).