You could add display: block;
to the :before selector.
p:before {
content: "This is inside the boxd and in the :before selector";
background: green;
color: white;
border: 1px solid transparent;
border-radius: 5px;
padding-right: 100px;
text-align: center;
display: block;
}
Here is a fiddle http://jsfiddle.net/4XY2R/
Assuming this is what you're trying to do.