This article is from the Aerobics Fitness FAQ, by firstname.lastname@example.org (Robles) with numerous contributions by others.
The best step height is the MINIMUM necessary for you to get
a good workout. Maintaining range of motion and adding
propulsion to your moves can increase the intensity of your
workout much more than adding a riser.
Keeping the above in mind, step height depends on several things -
fitness level, current stepping skill, and the degree of knee
flexion when the knee is fully loaded while stepping up. At no
time should the knee joint of the first leg to step up flex beyond
a 90% angle. Reebok is now saying that 60% is even better.
Deconditioned individuals or beginners should begin on a 4"
platform. As you improve, you may add risers to increase the step
height making sure not to exceed the 90 degrees of knee flexion.
The most popular step heights are 6" and 8".