I have noticed that imap.index.data are often too large files (several MiBs). Is there a way to limit their size or to shrink dimension or to assign another directory where those file are stored?

Default MAIL environment to use when it's not set. By leaving this empty

dovecot tries to do some automatic detection as described in

doc/mail-storages.txt. There's a few special variables you can use:

%u - username

%n - user part in user@domain, same as %u if there's no domain

%d - domain part in user@domain, empty if user there's no domain

%h - home directory

You can also limit a width of string by giving the number of max. characters

after the '%' character. For example %1u gives the first character of

username. Some examples:

maildir:/var/mail/%1u/%u/Maildir

mbox:~/mail/:INBOX=/var/mail/%u

mbox:/var/mail/%d/%n/:INDEX=/var/indexes/%d/%n

-- Sven