Here space is the primary concern.
There's a couple of built-in settings in iShowU HD that do this. Click the Output video button in the toolbar and choose Email. Three options are available. Choose one to suit.
The essence of the presets is this: reduce the frame rate down, compress directly into H264 and scale the video down as well. The combination of all three provides videos that still serve the purpose of explaining something to someone, while at the same time not being overly large to send via email.