I don't know what Comcast's size limit is per email, but I
am guessing it won't be any bigger than 5MB for a file
going through their mail servers.
The mailbox limit is likely 25MB or so, so the single
email size could be as large as 10MB.
If the file is bigger than that, you're going to have to
find a different way to send it.
Send it over IM directly to the person, burn it to a CD
and give it to them, try shouting 1's and 0's at their
computer...