you can check the file lenght of the file that the user is trying to post to the server then display your message

try
{
   if(fileToBeUploaded > constantMaximumFileSize)
   {
      //display your pretty message here
      messageLabel.Text = "Hey! You can't upload a file more than " +
                       + constantMaximumFileSize;
   }
   else
   {
      //do your upload code here
   }
}
catch(HttpException httpEx)
{
   //HttpExceptions is an exception that occurrs during the processing of HTTP requests
}

HTH