resize images with php

the image size used for the single product image is controlled by the theme.
If you have a decent theme, it will allow you to alter the size in the customizer:
https://docs.woocommerce.com/document/image-sizes-theme-developers/

the way image sizes work is wp creates a copy of the original image for each size that is defined.

If you change the configured size, you can use https://www.ads-software.com/plugins/regenerate-thumbnails/
to update the sizes of existing images

bheadrick offers good advice here. We haven’t heard back from you in a while, so I’m going to go ahead and mark this thread as resolved. If you have any other questions please feel free to start a new thread.