By default the size limit of a message is set by Postfix at 10240000 bytes (roughly 10MB). You can check the actual message size limit with the following command:
postconf -n | grep message_size_limit
If no value is returned, then the default value is used. You can check this value with:
postconf -d | grep message_size_limit
To set the message size limit:
postconf -e 'message_size_limit = 104857600'
This will set the size limit to (roughly) 100MB.
Restart Postfix to activate the new size limit.