To change the message size limit, execute the following command:

sudo serveradmin settings mail:postfix:message_size_limit = number

To disable the message size limit, execute the following command:

sudo serveradmin settings mail:postfix:message_size_limit_enabled = no

Note: The size limit can be reinstated by using the same command with a value of yes in place of no. You can check the current message size limit value with this command:

sudo serveradmin settings mail:postfix:message_size_limit