Please - no offence - no irony in this at all - just bit of lack of phantasy (in my department). BUT
If You don't like it scaled up to fill sideways and You don't want black borders.
Is it stretched sideways You want . Short fat people and all scaled just sideways - or.
What do You want and wherefrom would that information be retrieved that should fill the black borders ?
What's not recorded can't be created magically as far as I know.
So I guess that You need to explain much more how You think it should look and with what material it would be created.
Yours trying to get a picture of this - Bengt W
In more advanced video editors one can put a picture as background and sort of frame the vertical video clip or make two video clips side by side to fill.
May be one with skill in iMovie'08 to 11 also can do this but I'm lost over to FinalCut where this is easy (but initially learning and get confortable in FCE/P was quite a steap in-step in learning)
Yours much happier to understand Your dilemma: Bengt W
iMovie assumes you will be watching the movie on a device in a 16:9 or 4:3 aspect ratio. This covers almost all TVs and almost all computers. Also covers most iPads and iPhones.
To watch a movie on your HD TV without any black bars, you would have to rotate your TV set 90 degrees, and most people do not want to do that.
However, there are some legitimate uses for odd shaped movies. One example of this is a web page. You might want a 1 inch by 5 inch movie playing in the left margin of your web page for example. To do this, the best tool is Apple's QuickTime Pro Version 7. It is available from Apple's website for about $30.
There is a QuickTime Pro discussion group here in Apple Discussions for detailed questions on QuickTime Pro.