How to check and change postfix attachment size (topic for posterity)
I had problem with attachments in my postfix configuration, so I checked what is the value of message_size_limit, you can do it by:
postconf | grep message_size_limit
Result of this command wiil be size of a message which can be accepted via (smtp/imap/pop3) in bytes.
To change that type:
postconf -e message_size_limit=20480000
In that case, message will be limited to 20MB.
After setting the correct value, you could restart postfix service.