Gmail has a limit on the message size, which, I believe, is 15 MB. I don't know what it is for Yahoo. As @KieSeyHow, most e-mail services have such similar limits.
As an aside, I think sending videos as an attachments is barbaric and atrocious, and should be avoided as much as possible. In some cases (small university-hosted accounts for students, some greedy e-mail providers, etc.) it may even fill up the recipient's mailbox, resulting in the subsequent messages bouncing.
There are cloud storage places like box.com, dropbox, etc. that give you some storage for free (typically 5-10 GB), and you can upload your videos there and share them by the direct link.
That's what I do when I need to share large photos in their full original size (some 5-10 MB per photo).
I use Youtube for sharing videos (making them private to share them only by the direct link) -- that provides a much more convenient interface for viewing, and no toll on the recipient's storage (e-mail, phone, tablet).
