This section will scale images based on the size of the screen, so there is no particular size that will display in full for all screens. A good starting place is 1400px by 640px, as recommended in the theme customizer. However, since all Pixel Union's Shopify themes feature Responsive Images, you can add the highest quality image files you have and the right image size will be displayed for whatever device the customer is using.
It is important to note that regardless of the size of your images, cropping on certain screen sizes will likely occur, so using images with text or logos is not advised.
Landscape-oriented or square images tend to display more content. Conversely, portrait-oriented images will be cropped more and display less content, as you can see in the screenshot image below.