Click the Router/SMTP – Restrictions and Controls – Restrictions tab.
Maximum message size = The maximum message size in KB (thousands of bytes) the server accepts. The Router rejects any messages that exceed this size for both transfer and delivery. The default is 0 KB, which does not limit message size.
Note: you might also want to set up a Mail rule that states any mail over this size is also not accepted this will stop Journal messages etc being sat DEAD in the mail box.