We recommend a minimum of 4000px wide. This gives enough width for high density screens (2x).
Remember header images get cropped at the sides as the screen size shrinks (e.g. mobile). So we highly recommend you don't put text on the image as it'll get chopped off. Instead use the Overlay content and the Focal point to the keep the main context in view.
We recommend a minimum of 2000px wide. This allows adequate zooming when hovering over the image on the product detail page.
The product detail image itself has maximum dimensions of 1140px wide by 1710px high. This is the size for high density screens (2x).