1)Set global limits in file /etc/php5/apache2/php.ini:
upload_max_filesize = 32M
post_max_size = 32M
2)Set local limits in /usr/share/roundcube/.htaccess
php_value upload_max_filesize 16M
php_value post_max_size 17M
php_value memory_limit 256M
P.S. Perhaps you will need to adjust your Postfix SMTP server message size limit in /etc/postfix/main.cf
message_size_limit = 33554432