效果如下：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>JS Bin</title>
<style>
span{
display:inline-block;
width:4.5em;
text-align:justify;
font-size:20px;
overflow:hidden;
height:20px;
line-height:20px;
}
span:after{
content:"";
display:inline-block;
width:100%;
}
</style>
</head>
<body>
<span>一字</span><br/>
<span>两个字</span><br/>
<span>有四个字</span>
</body>
</html>
