Re-sizing blues!

I've been on a tear the last week or so, resizing all of my avi's so that they have a more full "letterbox-type" look to them. I'm not very scientific about it and I really don't know if I've been doing it all right using AVIdemux, but they've come out pretty good.

My question though is...Are there some video files that just have their limits as far as re-sizing? I say that because whenever I start working on a file, I try some settings, test out the video, then try some more, eventually until I get to where I want it, which is filling up WMP screen.

See I have found that by doing that, when I play it back on my home machine, the real letterbox vids aren't so skinny anymore and the 4:3's look pretty good too.

But out of about 20 files I've worked with so far, I've had two of them that I kept trying to stretch but, in the end, they just would not look like I wanted. This baffles me!