Approximately 30 MB.
The answer is approximate. MIT's email system has a maximum message size of 40 MB for both incoming and outgoing messages. This counts everything that goes into a message - your text, plus all the mail headers, plus the all the attachments, needs to fit within the 40 MB limit.
Files expand a little when they are encoded to be attached to email, so the practical limit is a little lower, to give room for the file to grow. 30 MB is a good rule of thumb - a file under 30 MB file will grow when encoded for email, but should still fit within the 40 MB limit for "total message size".
|Please note that this is MIT's setting; your recipient's limit may be set much lower, causing your email to bounce back.
GMail limit is 25 MB http://mail.google.com/support/bin/answer.py?hl=en&answer=8770