@Turritopsis_Dohrnii,
In Postfix, the “message_size_limit” is by default set to “1024000” (approx. 10MB). This size includes every aspect of the message however including: headers, message, and attachments, etc.
Most servers today support a message size of at least 25MB these days, so to increase the size you can execute a few commands from the command line.
sudo postconf | grep ‘^message_size_limit =’
This command will tell you what the current “message_size_limit” is which likely will result in the initial value from the first part of the question.
Next, to change the value execute:
sudo postconf -e ‘message_size_limit=25600000’
This will change the value in the Postfix configuration.
Once again, you can run:
sudo postconf | grep ‘^message_size_limit =’
Confirm that the value has been changed correctly.
Finally to apply the changes so that Postfix uses the update, run:
sudo systemctl restart postfix
Report back if this addresses your concern, or if you require further assistance.
*** the above instructions increases “message_size_limit” from 10MB to 25MB ***
It’s quite possible this change could be made from within Webmin via:
Webmin > Servers > Postfix
However I’m old school, so I prefer doing stuff via the command line 
Good luck!