Postfix set limit of user mail

[wang]

HELLO!
        How can i set the limit of user mail at /var/spool/mail
        Because i have some message at th log
        
        to=<[EMAIL PROTECTED]>, relay=local, delay=1,
        status=bounced
        (cannot access mailbox /var/mail/webmaster for user webmaster.
        error writing message: File too large)
        
        MANY THANKS
        
        wang