All depends what your server host allows you to upload max size as? Makes no difference setting 15mb upload size if your hosting limits upload size to less than that. Have you checked "PHP Information" page that your host sets via Admin CP, look at upload_max_filesize on it
Adding those lines to an htaccess file makes no difference half the time, it will not override the php.ini file server host sets for max-upload size, you would need to contact your host and ask them to change it in INI file. Don't be fooled into thinking that anything you add in an htaccess file overrides things, most of time it won't work.