Let's say something like this is being returned when attempting to send an email using a Postfix email server.

5.3.4 message size limit exceeded

By defaut, the size limit is 10240000 bytes, which can be seen with the following command.

~]# postconf message_size_limit
10240000

If you want to change the default size limit, you can set message_size_limit in /etc/postfix/main.cf.

message_size_limit = 20480000

And then restart Postfix for this change to take effect.

systemctl restart postfix