Im trying to use some 16x9 mattes I downloaded on here on a production, but when I load the file on the memory stick, and they apply it using the mix function, it appears in the middle of the screen, very small, rather than filling the screen.

Any ideas?

Are the images 640x480? How many pixels per inch?

Yes, they are the correct pixel size. 720x576 (pal)

Very strange.