If you make a quick script that just includes:
<?php
phpinfo();
?>
And run it from the server, you should see at the top the location of php.ini. Copy php.ini-dist to this location (renaming it to php.ini) and then you can change the upload filesize. If this is run as a module to any webserver, the webserver will need to be restarted then.
Hope that helps!
Chris King