By default Rackspace Cloud Sites upload limit is set very low at just 2MB lucky this is easy to change all the way up to around 100MB per file. For the sites you want a larger PHP upload limit on simply add the following line to the .htaccess file
php_value upload_max_filesize 25M
You can also put this in a .htaccess file at the route of your account to apply the new limit to very site you host.
You can check if the change was successfully applied on the WordPress upload pages or by creating a PHP page with the following and browsing to it
<?php phpinfo(); ?>