<input type="file" onchange="viewSize(this)" />
<script type="text/javascript">
    function viewSize(obj){
        if(document.all){
            obj.select();
            var fso = new ActiveXObject ("Scripting.FileSystemObject");//IE比较麻烦,安全性提示和各版本不兼容
            var f = fso.GetFile (document.selection.createRange().text);
            document.selection.empty();
            alert(f.Size);
        }else{
            var file=obj.files[0];
            alert(file.size);
        }
    }
</script>