The upload server behavior will not be able to limit the file uploaded until it has been sent to the server, so you would not get any type of notice until after the file has been uploaded to the server.
If you do not want your file uploading for a long time first you should set the timeout to a smaller value with this code:
<?php set_time_limit(); ?>
You would then put in the number of seconds you would like the script to run until the time expires in the parenthesis.