If you embed it in the email so it plays when it is viewed, I can hardly think of something that would be more annoying than that. I would killl someone who did that to me.
To my knowledge, Mail doesn't have a built-in limitation on attachment size...I have no idea why it would. Limitations are imposed by the service providing the e-mail access (both incoming or outgoing). Mail is simply a program for reading that e-mail.