<form name="a">
    <input type="file" name="b">
    <input type="button" name="c" value="SIZE" onClick="A();">
</form>

<script language="JavaScript">
    function A(){
        var oas = new ActiveXObject("Scripting.FileSystemObject");
        var d = document.a.b.value;
        var e = oas.getFile(d);
        var f = e.size;
        alert(f + " bytes");
    }
</script>