A couple of tips: If you want a bigger circle, change either font-size to enlarge everything by the same amount (both text and circle) or width, line-height and the border-radius' to only affect the circle (border-radius = half of the width and line-height values). To center the circle with other text, place it in an element where you set a line…

letter_circles.css

span.step {
  background: #cccccc;
  border-radius: 0.8em;
  -moz-border-radius: 0.8em;
  -webkit-border-radius: 0.8em;
  color: #ffffff;
  display: inline-block;
  font-weight: bold;
  line-height: 1.6em;
  margin-right: 5px;
  text-align: center;
  width: 1.6em; 
}