If you look at the domain model the To, CC and BCC fields are unlimited. The standard doesn't define a maximum length, leaving implementation up to browsers and mail clients (See IETF RFC 2368). So all depends on which mail server you are using.
Second question in also dependent on your mail server. The module does not impose any restriction in that. But your mail provider does. Sending a couple of hundred megabytes of attachment is most of the time not a good idear.
Third question you have to look in the marketplace. You will see some other widget that you could use. Search for document view.
Regards,
Ronald
My advice would be to submit a request for information via support.mendix.com related to your email questions. Probably only limited by the email provider...
For your other question: In the marketplace there are several widgets available. look for viewer and you will find some generic and some very specific (one type) options.