I hope you are well today and thank you for your question.
If you want to change the image size on the single product page then please make sure to change it in the option “Single Product Image” of Woocommerce plugin settings.
Then regenerate the thumbnails using the following plugin.
After changing the size of the image could you please try adding the following CSS code in the Custom CSS option of your theme on the below path.
Admin Area -> Appearance -> Theme Options -> Other -> Custom CSS