At 5'4", I wouldn't use any higher than 14" for stepping up on (higher for sitting). 12" may even work better for you. It all depends on the angle of your knee when you put your foot on the step. It shouldn't be any tighter than 90 degrees (a right angle). Any tighter than that, and it puts too much stress on the knee. TO add intensity without adding height, be sure to push through the heel of the foot on the step.