I’ve attempted to follow the WooCommerce recommendations here and here with no success. The single product image in WooCommerce is blurry and the image that is magnified is fine. Somehow I’m not accessing the right image/thumbnail. My settings in Customizer > WooCommerce > Product Images are 600px, 300px, Uncropped
After turning off all plugins I discovered a snippet of php (Code Snippets) that I had inserted which defined single product images at the wrong size. I had placed it there before discovering the WooCommerce controls in the customizer. Once I deactivated the snippet, things cleared up.