Keep in mind that the maximum Shopify product image size you can upload on Shopify is 4472 x 4472 pixels with a file size of 20 MB.
However, most of the time, this is an overkill. You won’t need such big images for your store, and they can slow down your loading speed.
Instead, Shopify recommends using 2048 x 2048 pixels for square product photos. You can use the JPG or PNG format to have a great balance between high quality and fast site loading speed.
All and all, product images shouldn’t be smaller than 800 x 800 pixels. If not, functions like zooming in might not work very well.