http://jsfiddle.net/MafjT/
You can use this css
span {
display: block;
height: 60px;
width: 60px;
line-height: 60px;
-moz-border-radius: 30px; /* or 50% */
border-radius: 30px; /* or 50% */
background-color: black;
color: white;
text-align: center;
font-size: 2em;
}
Because you want a circle, you need to set the same value to width, height and line-height (to center the text vertically). You also need to use half of that value to the border radius.
This solution always renders a circle, regardless of content length.
But, if you want an ellipse that expands with the content, then http://jsfiddle.net/MafjT/256/
Resize with content - Improvement
In this https://jsfiddle.net/36m7796q/2/ you can see how to render a circle that reacts to a change in content length.
You can even edit the content on the last circle, to see how the diameter changes.
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