Maybe you already know how? De Prez Allen Laudenslager writes, "I was trying to send a video clip as an email attachment and it shows up as a thumbnail, way too small to be viewed, as I am composing the message. That really threw me! I checked with Apple and that's normal and when you actually send the email the clip is about 2 inches square and big enough to view. If you save the attachment, then all the features of Quicktime become available.
"After I sent the email, I looked in the sent folder and the video clip looks right (about 2 inches sq) in that copy of the message."