Picturemove effect, diff sizes

I searched through the forum a bit but didn’t find a post specifically about this so please forgive me if I missed it!

I like the picturemove_user effect a lot and the tutorial is great, but what about the ratios of the background images to the movie width?
ie: for a movie that is 500px in width, what size should my static background be and what size should the moving background be to keep a relatively continuous and smooth scroll movement?