You can do that directly at Exchange. You can set it globally (all mailboxes) or individual users.
Start Exchange System Manager.
Expand Global Settings.
Right-click Message Delivery, and then click Properties.
Click the Default tab to configure the global settings.
Start the Active Directory Users and Computers snap-in, and then locate the user account that you want to configure.
Right-click the user’s account, and then click Properties.
Click the Exchange General tab, and then click Delivery Restrictions to set the maximum size that you want to permit.
Since this is an Exchange 2007 account you can create the rule you want as a transport rule. Open the Exchange management console. Expand Organization configuration and click the hub transport role. On the right under actions select new transport rule. You can choose that messages sent to certain people (find the name of the mailbox you want this to apply to after selecting this by clicking the word people in the bottom center pane) with attachments over a certain size limit and set the limit in the bottom pane, click next then select “send bounce back message with enhanced code”. Set the message you want in the bottom pane by clicking the default message in blue. Finish creating the rule and voila, you will send a customized message to senders.