This has nothing to do with ISPConfig, it's a setting of your Postfix.
postconf -e 'message_size_limit = 20000000'
postconf -e 'mailbox_size_limit = 100000000'
This will accept messages of sizes up to 20000000 bytes, and the max. mailbox size is then limited to 100000000 bytes. Use whatever values you like. Restart Postfix afterwards: