How large a message can I receive or send?

You can send messages up to 50 MB. Assuming there enough disk space in your mailbox, you can receive messages up to 50 MB.
The message total size included the message size + the attachment(s) size.
The size limit for an outgoing message is set to 50MB, which is rather large compared to what other mail providers allow.
If your message is larger than that, try to send multiple messages with smaller attachments, or send the attachment using a file sharing service like Dropbox.com