Try:
<head>
<script>
function GetFileSize()
{
var myFSO = new ActiveXObject("Scripting.FileSystemObject");
var filepath = document.upload.file.value;
var selectedfile = myFSO.getFile(filepath);
var size = selectedfile.size;
alert("File size:"+ size + " bytes");
}
</script>
</head>
<body>
<form name="TestUpload">
<input type="file" name="file">
<input type="button" value="Size" onClick="GetFileSize();">
</form>
</body>
</html>