How to use Avidemux to resize an HD video so that it is small enough to upload to a website or email.
Using Ubuntu, open Avidemux by going Applications > Sound and Video > Avidemux.
Load a video
Trim clip (if necessary)
When the video is open, you will see a preview of it. If you need to trim the clip, set a beginning and end point by moving the slider then clicking the A (start) and B (end) buttons.