Greetings,
There is no way with this model to configure a default size that applies to specific paper sizes. You can configure an overall default size, to limit the total size that emails can be sent as in general, but not for only legal size - that setting would apply regardless of what paper size you use. If you need help with this, I suggest reading this article or contacting support for further assistance.
Additionally, settings like maximum attachment size can typically be modified on most modern email servers. If your email server is hosted locally, you may be able to simply contact your email administrator and ask them to increase the maximum email attachment size, which would resolve the issue entirely. If your email server is not locally hosted, you can contact the company that hosts email for you, but I don't know that they would be able to adjust those settings.
