Can I change the Message Transfer Agent (MTA) size limits on a schedule? I want to set the maximum MTA size to 1MB during office hours and to 3MB for off-hours.

Unfortunately, you can't schedule changing MTA size limits. Exchange stores the maximum MTA message-size limit as a value in the Exchange Server 5.5 directory. Therefore, you'd need to write code that uses the Directory Service API (DSAPI) to change the message-size limit, then run the DSAPI on a schedule that you set.