There are a couple of answers. First You could record in a different format on your camera. Most have the option of widescreen.

Second you could record them to your computer and open them with a video editing software. Then save them as another format and in a different size.

Youtube says the less a video is re-encoded prior to uploading, the better the resulting YouTube video quality. Getting Started : Optimizing your video uploads - YouTube - Broadcast Yourself.

YouTube player automatically adds black bars so that videos are displayed correctly without cropping or stretching. the new player shows 16:9 widescreen anything else will produce black bars down the side. See more about it at ( my arse )

and some video tips
( link removed - mw )