In Exchange 2003 the Sending message size and Receiving message size settings for the Exchange organization are set to 10240 KB. This default setting applies to new installations, or upgrades from Exchange 2000 in which no default size was set.
If you choose to change this size by going to ADUC right click a user >properties>choose the Delivery Restictions tab > You can then choose sending message defaults (see above) or set your own size. The user will get a default message back saying cannot send as you have exceeded the set limits.
In routing groups – You could use different delievery shedules. Delivery options tab tick “use different delivery times for oversize messages” – set the size in KB which exchange will classify as oversize then choose customise and send out of work hours works great.
may also help http://technet.microsoft.com/en-us/library/bb310771.aspx