Hello, @Martine,
You can set the 4 products next to each other in a row for the shop page also you can set the product image sizes.
To set the 4 products simply go to the Dashboard >> XSTORE >> Theme Options >> WooCommerce >> Product Catalog >> from there you can set the 4 products per row see image: https://postimg.cc/crNnq883
To set the product images size navigate to the Dashboard >> XSTORE >> Theme Options >> WooCommerce >> Product Images and from you will be able to set the product images size check the image: https://postimg.cc/5XXFdcmH
These are the options in the theme. You cannot set the product limit per row on the specific resolution you want. Changes will work on other devices like tablets and mobile automatically.
For the single product page product image if you set the image size using custom CSS it will destroy the product image pixels. As I told you how to set the product image size you can check this option otherwise use the high-resolution product images to cover up the empty space.
Note: whatever size of the image you use kindly then use the same size for all the images.
Regards 8Themes Team.