I tried this but keep getting following error when I put this into a content editor web part.
<script type="text/javascript">
var FSO = new ActiveXObject("Scripting.FileSystemObject");
var input_File = document.aspnetForm.ctl00_PlaceHolderMain_ctl01_ctl02_InputFile.value;
var uploaded_file = FSO.getFile(input_File);
var file_size = uploaded_file.size; //size in bytes
alert(file_size + " bytes");
</script>
Message: Automation server can't create object