I tested your code using a simple test and it works fine...
<!
DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="nl" xml:lang="nl" xmlns="http://www.w3.org/1999/xhtml">
<head>
<script type="text/javascript" src="js/jquery.js"></script>
<script type="text/javascript">
$(function () {
alert($("form[name=giveLocaForm] :checkbox:checked.selected-track").size());
});
</script>
</head>
<body>
<form name="giveLocaForm">
<input class="selected-track" type="checkbox" checked />
<input class="selected-track" type="checkbox" />
<input class="selected-track" type="checkbox" checked/>
</form>
</body>
</html>
Looks like it's something else in your code.