change dovecot mbox localtion

[Charlot W]

hi,
i use:
dovecot 1.2.7, sendmail and unbuntu to serve my mail service.
recent my disk space is not enough, so i want to mange my mail in two
ways:

*1. change the mail location:*
i use:
mail_location = mbox:/var/mail:INBOX=/var/mail/%u
when i changed it to
mail_location = mbox:/home/mail:INBOX=/home/mail/%u
but the mail still write to the old path

*2. limit the mail file size*
i want the mail file reduce when it reach to 100MB


how can i config dovecot/sendmail to do in last 2 ways?

thank you all:)