Paralax effect image size

I’ve been playing about with the call to action widget and the background image parallax effect.
Could you tell me what the ‘window’ space size is please?
I’ve been trying to get my images so they fit by the width but they keep getting cropped. I’m not sure what the largest width and depth can be and which one over rides the other.

Hope that makes sense!

After 8 hours of trying everything I could think of it was the page template!

Front page not full width! Sorted.