Wrong query in dovecot-sql.conf prevents dovecot from managing quota
With standard "perfect" configuration dovecot don't create maildirsize so no quota management, no quota warnings, no mailbox size in squrrielmail/roundcube.
Looks like dovecot 1.2.7 (and probably 1.2.6) incorrectly interpret quota=maildir:storage=xxx, correct parameters are quota=maildir and quota_rule=*:storage=xxx
Problem is solved by change in user_query in dovecot-sql.conf. Correct query is: SELECT email as user, maildir as home, CONCAT('maildir:', maildir, '/Maildir') as mail, uid, gid, 'maildir' as quota, CONCAT('*:storage=', floor(quota/1024)) AS quota_rule, CONCAT(maildir, '/.sieve') as sieve FROM mail_user WHERE email = '%u' AND disable%Ls = 'n'