The following code shows how to set width for input-group-addon.
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" type="text/css"
href="http://netdna.bootstrapcdn.com/bootstrap/3.0.0/css/bootstrap.min.css">
</head><!--from w w w .ja va 2 s. c o m-->
<body style='margin:30px'>
<form>
<div class="form-group input-group">
<span class="input-group-addon">X</span>
<input type="text" class="form-control" id="svX">
<span style="width: 55px;" class="input-group-addon">km</span>
</div>
<div class="form-group input-group">
<span style="width: 55px;" class="input-group-addon">X</span>
<input type="text" class="form-control" id="svX">
<span class="input-group-addon">km</span>
</div>
<div class="form-group input-group">
<span style="width: 55px;" class="input-group-addon">X</span>
<input type="text" class="form-control" id="svX">
<span style="width: 55px;" class="input-group-addon">km</span>
</div>
</form>
</body>
</html>