What's the maximum size of an email to be sent by Apple's Mail program?

Not determined by Mail, but by your ISP or email provider. Mine doesn't allow more than 20 MB. I'm fairly certain at 2.9 MB, you'll be fine. You know, if you have a file that's really large, zipping it using File>Compress, can usually, but not always, reduce it significantly.

susanzimm wrote:

Is 2.9 MB too large?

No, in terms of Mail.app. As WZZZ said, this limit is set by your ISP.

Yes, in terms of netiquette. Google it. Any file > 1MB is, IMHO, too big for e-mail. Nowadays, this rule may be more relaxed, because more people have broadband. You should check with your intended recipient.