Setting mailbox quotas?

my brother wants an email account on my domain, but I dont want to give him unlimited space. Is it possible to set up a quota for a mail account?

Sure. Mail -> Mailboxes -> edit (for the appropriate user).
Note that quotas can’t be set or calculated separately for mail if the user is also an ftp/shell user - they show up separately and can have different passwords, but the quota is shared between the two.