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After you follow Steve's instructions, remember that video is horizontal, and if you Rotate your Clip, you will likely have black bars on either side of it.
You can leave those black bars, or can Scale (same instructions, as Steve gave, but instead of Rotation, you would go to Motion>Scale) that footage, but the quality will go down. Also, you will be "effectively cropping" the top, bottom, or both, to fill the horizontal Frame Size. You can determine what is effectively cropped, by using Motion>Position on that Clip.
I normally leave the black bars, in such a situation, or create an "abstract background" to hide the black bars, and form some "framing elements" to cover them. This article gives some tips on creating and working with an abstract background: http://forums.adobe.com/message/2471303#2471303
It mentions "for SlideShows," the most common place that one works with portrait oriented Assets, but works fine for Video too. That technique is often used by broadcast networks, when their feed material is of a differient orientation, or Aspect Ratio, than their feed. FoxNews creates a "ghosted," heavily blurred version of the Video, Scaled up to fit (quality does not matter, since the footage is ghosted and blurred for the abstract background), and then just do a PiP (Picture in Picture) for the actual footage, which will be smaller than the screen size.
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