<script type="text/javascript">
     document.getElementById("Button1").onclick = function () {
         var fl = document.getElementById(‘FileUpload1‘);
         if (fl.value.length <= 0) {
             document.getElementById(‘Label1‘).innerHTML = "请先选择要上传的文件!";
             return false;
         }
         else {
             if (fl.files[0].size > 1024 * 1024 * 4) {
                document.getElementById(‘Label1‘).innerHTML = "文件过大，不允许超过4MB大小！";
                 return false;
             }
        }
    }
</script>

if (FileUpload1.PostedFile.ContentLength > 1024 * 1024 * 4)
         {
             Label1.Text = "文件过大！";
             return;
         }