There is not a limit defined by email standards outlined in RFC (Request For Comments) memorandum's. However, due to the cost of sending very large files, most email services set a limit of around 1mb-10mb for an attachment. Mail protocols also have a problem handling large files, and they usually require an attachment to be broken down into separate parts to be sent. Attachments have had a bad press because many have included viruses; therefore, it's become an essential part of an mail client, that it has the ability to scan attachments for viruses.
The problem with email - in relation to attachments - is that messages are usually passed through a number of MTAs (mail transfers agents) before they arrive to the receiver. Mail services, such as Gmail/Yahoo!, usually, do not have control over how these transfer agents deal with the messages passed through them. In reality, very large files, of say 200-500mb, often have problems navigating through these transfer agents. Most mail services have stated that they can only reliable transfer files of 10mb, and even if they wanted to send larger files, they could not reliable do so.
However, as of 2012, it must be noted that free mail services are increasing the size of the attachments they send. Therefore, as time passes, it may be possible to reliable attach files of 500-1000mb to an email.
The above error message was prompted when trying to attach a file larger than 50mb to an email in the mail.com email service.