what you need to do is create a file called "phpinfo.php", and in the file only write the code:
<?
phpinfo();
?>
upload it on to your site and open it. then you will get a list of all the PHP variables attributed with your site. Look for one called upload_max_filesize. it is probably 20MB. You need to talk with whoever is able to access those (most likely tech support tickets @ cpanel work), and tell them to change it to whatever you wish. they will do it.