High-resolution "Retina Displays" render images at twice the pixel density of regular displays. This means that standard images often look fuzzy or blurry on a retina display. In order to account for these extra pixels, an image needs to be displayed compressed at half the size. For example, in order for a 200px wide image to be displayed crisp on a retina display it would need to be displayed at 100px wide.
The Retina theme will automatically use larger product image sizes for your various products to account for this compression. This allows for your product images to be displayed beautifully crisp across all types of screens at various sizes.
If you are using the sidebar on the product page, it is recommended to upload product images at least 860px in width.
If you are not using the sidebar on the product page, it is recommended to upload product images at least 1160px in width.
As a rule of thumb, it is always recommended to upload images as large as possible. Shopify will automatically resize your images to various different sizes to be used throughout the theme.