Ok some severs hide the .htaccess its in the root of the roundcube folder just make a new .htaccess and put in this code:
# AddDefaultCharset UTF-8
php_flag display_errors Off
php_flag log_errors On
php_value error_log logs/errors
php_value upload_max_filesize 2M
<FilesMatch "(\.inc|\~)$|^_">
Order allow,deny
Deny from all
</FilesMatch>
Order deny,allow
Allow from all
Change the "php_value upload_max_filesize 2M" line for the upload limit