Hi,

There is no way for you to check the file size before the user attempts to upload. Otherwise it would have been a serious security hole where website owner can write pages to automatically gather file size information from user's computer without user having to do anything.

The most you can do is to set the uploader's MaxDataSize to restrict the file size. This allows the uploader to check the request header size. The check occurs after user clicks upload button, but before the actual file is being transferred.

Thanks!