Did you see the quota indicator? People will see there how big they are getting.
It is not so easy to do this by a cron, because you must access Roundcube index.php to have all built-in classes/methods available.
Basically you get counts by ...
$rcmail->imap->messagecount('INBOX', 'ALL', TRUE);
[INBOX has to be replaced by the folder name]
The list of all folders can be obtained by ...
$rcmail->imap->list_mailboxes(); [returns an array]
Roundcube remembers folder stats in $_SESSION.
I don't know, if this is already available when summary plugin jumps in. You have to test.
A general hint: Look in ./program/include. This folders contain most of the classes used by Roundcube. So you can easily find starting points there.