You can limit the size of incoming messages as follows:
Maximum message body size option
This option looks at the message body (not including attachments and headers) and truncates it if it's too big. Set this parameter in Content Administration using the Mail Settings window.
Maximum message size option
This option looks at the total size of the message (including attachments and headers) and truncates appropriately. Set this parameter in Edit POP Mailbox. From the Mail Receiver window, click Edit Mailboxes, then click the icon next to the appropriate mailbox.
Mail Receiver Rule - If body length is greater than (nnn)
This rule looks at the message body, but you cannot truncate the message this way. Instead, you can apply other actions, such as deleting the message if it is too big or sending an acknowledgement back to the user.
See About rules and rule groups and Attributes reference in Content Administration.