In short: the total message size limit (emails + attachments) is 30MB.
- The total of MB you can process through Flowmailer is 22.5MB;
- The total attachment size is 22.5MB, minus your email's size.
The 30MB does not take into consideration that emails are Base64 encoded. This encoding increases messages by 33% - so 30MB is 133% of what you can send through Flowmailer. To stay under this 30MB limit, a message size of 22.5MB is the maximum message size you can process through Flowmailer: 22.5 * 1.33 = 29.925MB.
You cannot increase this limit, but if you need to lower it for any reason, go to Setup > Sources & API > 'name of your source' > Edit Source.