I'm trying to create a full width bg slider whats the best size for images?

I'm trying to create a full width bg slider whats the best size for images?
So I had a problem with the default top section bg slider getting corrupted I posted another thread about. Solution, I'm deleting it and replacing it with a new bg slider.

What is the best size for a full width background slider to make all the images as many of my images are different sizes and I'll need to crop them to the same size so they look proportional.

Thanks
Joe