No limits, including any attachments.
Although there are no limits imposed by the system on the size of attachments we do encourage email users not to send large files via email.
You should also consider a user's quotas when sending large attachments. If the user's quota is insufficient then the mail with the attachment will be returned as undeliverable. If it is actually delivered then it may fill up a user's mailbox and they will be unable to receive further mail, or download the mail that has been delivered.