Is there an automatic way to have a PiP enlarge to the full frame size without having to stretch it?

I am combining videos, using PiP's, and when I first put them in, I have to stretch them out to fill the video screen. Isn't there an automatic way to fill the screen? The problem is that at the end, there will be thin black lines on some of the edges. I know I could over fill the frame, but I'd rather keep the original size.


Thanks,
Sean -