The Masonry section scales images based on the size of the screen, so there is no one-size-fits-all recommendation for images to display in full for all devices. That being said, a good starting place is 1400 x 640px, as recommended in the theme customizer.
Use the largest images you have
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.
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.