What are ideal dimensions for header slider images?

I would like to crop images for the header slider.
What are the actual or ideal dimensions, proportions, for the header slider images?

I work from a large screen – so my results get skewed a bit.

Enlarging such as the theme does to make images fit the slider is very convenient. However it sometimes cuts off, centers, offsets, changes on mobile, with a result that doesn’t work well or conflicts with the text of the slider.

Also, for example – on Android tablets the images render differently: they do not get enlarged to fill the screen, they render actual size.