how do I save a movie with 640x640 aspect ration for Instagram?

having issues saving movie clips in 640x640 aspect ratio for Instagram.

I've used a file converter to change ratios in the past, but for some reason they don't change both both the height & width to the needed 640x640 any more.

I figure there must be a way to work around the standard iMovie ratios offered from the Share & save to file options.....

Thanks