You can use jquery to control the width of the tooltip
check the below code i think it will solve your requirment
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<link rel="stylesheet" href="http://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css">
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script src="http://code.jquery.com/ui/1.10.3/jquery-ui.js"></script>
<script>
$(function () {
$(document).tooltip();
});
</script>
<style>
label
{
display: inline-block;
width: 5em;
}
.ui-tooltip
{
width: 210px;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<div>
<p> <label for="age"> Your age:</label><input id="age" title="We ask for your age only for statistical purposes."></p>
</div>
</form>
</body>
</html>