It is a length as much as a size thing (no pun intended). In my experiment, when I try to e-mail videos longer than 47 secs, a warning comes up: "Video Too Long To Send. Would you like to select a smaller clip from this video to email"? Tap "OK", and the video, by default, is trimmed to 47 secs and compressed to a size that varies between 4.6 - 4.7 MB. So even if your ISP or e-mail server allows attachments bigger than 4.7 MB, it appears that the trimmed video is stilled limited to ~4.7 MB.