A recommended size for a Shopify banner is 1800 x 600 pixels with a 16:9 aspect ratio for desktop, but this can vary by theme and page. It is best to aim for banner image dimensions between 1280 and 2500 pixels wide and 720 to 900 pixels high, with a 16:9 aspect ratio, but you should also check your specific theme's guidelines and consider smaller dimensions for mobile and collection pages.